Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 07/29/23
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1021 PM CDT Fri Jul 28 2023
.MESOSCALE DISCUSSION...
Issued at 1021 PM CDT Fri Jul 28 2023
A few HP supercells have been festering about from Winnebago
County east into the northern Chicago suburbs. Recent radar trends
suggest these cells are becoming increasingly outflow dominant
with consolidating cold pools. Despite thunderstorms in and around
the Chicago metro area and earlier passage of a lake breeze, sfc
obs suggest that the air mass over the Chicago metro has recovered
and is strongly unstable. The cluster of storms over the northwest
suburbs appear likely to bow east-southeastward across the heart
of the Chicago metro with damaging winds.
Farther west into Ogle and Lee Counties, very impressive rear
inflow jet evident on our radar with 70-80 kt inbounds noted at
5000 ft agl on KLOT 88D. Recent radar trends show the convection
associated with this rear inflow jet seemingly catching up with
the outflow ahead of it. If this trend continues, it would suggest
a increasing damaging wind threat as these storms track eastward
across the I-80 corridor. Air mass ahead of this line is extremely
unstable, with sfc dewpoints still around 80F, lending credence
the >3500 j/kg shown on latest SPC mesoanalysis.
If the western convection continues to catch up with the outflow
and becomes more shear balanced, then we would need to watch for
potential QLCS tornadogenesis with this western line. Particularly
where the outflow from the NW burbs bow intersects this second
western bow nearly perpendicularly (near DeKalb/LaSalle line as of
1020 PM).
- Izzi
&&
.SHORT TERM...
Issued at 325 PM CDT Fri Jul 28 2023
Through Saturday...
Key Forecast Message:
* Thunderstorms expected to develop near a cold front late this
afternoon this evening with an attendant severe and flash flood
threat
Heat and oppressive humidity this afternoon with dew points of
75-80F are expected to provide the fuel for potentially explosive
thunderstorm development as a back-door lake breeze enhanced
front sags south through this evening. While confidence as of this
writing remains on the lower side regarding exact trends, no big
changes to the thinking regarding severe and flash flooding
threats.
The main severe threats are expected to be scattered damaging to
locally destructive winds and isolated to scattered instances of
large hail. In addition, a brief tornado or two may occur near
frontal/outflow boundary intersections, particularly with any
bowing structures. A Flood Watch may be issued for portions of the
area over the next few hours as convective evolution and focus
area becomes more clear. Preliminary thinking regarding watch
issuance is generally the I-80 and north counties, with the flash
flood vulnerable Chicago and Rockford metros potentially in the
cross-hairs.
...General Expected Convective Development Trends into Early Evening:
Exceptionally high dew points near to locally just above 80F are
pooling ahead of the back-door front, making its way into far
northeast Illinois this hour. Recent AMDAR soundings and the
visual look to the Cu field suggest a capping inversion remains in
place. The "clumpiest" Cu resides from inland of the lake breeze
across northern Illinois west-northwestward into Wisconsin and
north central IA, where SPC/RAP mesoanalysis is indicating that
the cap may be on its way to eroding. In addition to the capping,
we`re lacking strong large scale forcing as of the 3pm hour, with
the most notable short- wave over the upper MS Valley and forecast
neutral mid- level height tendencies into the early evening.
May be signs of first CI over north central IA closer to the
aforementioned short-wave and near the cold front. Expectation
locally is for the back-door cold front to have CI once the cap
breaks given enhanced low-level convergence, plus upstream
convection to the west/northwest to fill in and gradually sag
south with the front this evening. Lake enhanced frontal
convection is forecast to develop in the 4-6pm timeframe, with
outflow gradually expanding the footprint southward, plus
additional convection northwest of the area likely to move in
toward and beyond 7pm for areas far inland of Lake Michigan.
...Severe Weather Threat:
Steep mid-level lapse rates and the swamp-like air mass are
contributing to very large MLCAPE up to 4000-4500 J/kg per recent
mesoanalysis. This is a "powder keg" sort of scenario in which
updrafts can quickly become severe thunderstorms in short order.
Effective deep layer bulk shear up to 30-40 kt with extreme
instability is sufficient for a severe threat, with storm mode
modulating residence time of variance threats. Downdraft CAPE
(DCAPE) pushing 1500 J/kg with steepening low-level lapse rates
suggests clusters becoming outflow dominant and potentially
congealing into bowing segments that would transition to more of a
damaging wind threat. Prior to that, the extreme instability and
steep mid-level lapse rates paired with supportive wind shear will
support both downburst damaging winds and large to potentially
very large damaging hail.
While damaging winds and hail are expected to be the primary
severe threats, we`d be remiss to discount any tornado threat
through a portion of this evening. 0-3km MLCAPE is forecast to
grow to up to 150 J/kg inland of the lake breeze front, so pooled
vorticity near the front introduce a concern for boundary
interaction related tornado-genesis. While low-level shear and
effective SRH are currently sub-marginal, effective SRH may
briefly increase to up to 200 m2/s2 early this evening. With the
sagging front, outflow boundaries, and increased low-level SRF in
play, we`ll need to monitor for quick spin-ups, especially on
bowing segments interacting with aforementioned boundaries.
We`re continuing to message the main window for the severe threat
to be from shortly after CI late this afternoon through about
midnight CDT tonight.
...Flash Flooding Threat:
Moisture pooling and advection streaming toward the sagging
frontal boundary of an already exceptionally moist air mass at the
low levels is expected to contribute to PWATs of up to 2-2.25" by
early this evening. With some localized areas in northwest Illinois
seeing over 2" of rain last night, and the ever-present vulnerability
of the Chicago and Rockford metros to flash flooding in these
areas, concern is certainly there for corridors of more than
localized flash flooding. Conceptually, this set-up checks some of
the key boxes for a frontal flash flood archetype, with potential
for back-building and training convection north of the sagging
cold front. The 12z HREF LPMM notably features some pockets of
3-5" of rain, with ensemble max hinting at upside potential being
5-7" amounts where repeated rounds of intense convection occur.
See WPC`s recent MPD for additional thoughts on this. We`re
looking at a decent chance of issuing a Flood Watch once
convective trends have shown their hand more conclusively.
Later Tonight-Saturday:
It`s unclear how long more intense convection will persist
tonight, but generally showing higher PoPs stair-stepping south
along with thunder chances with time overnight. We`ll have a large
reservoir of instability to work over, but sometimes reinforcing
outflows and force the effective surface front and front aloft
more quickly southward, so PoP trends will undoubtedly need
refining this evening.
On Saturday, most of if not the entire area is expected to be dry
with increasing sun from north to south. Minor exception to this
may be south of the Kankakee River where lingering instability
axis will take the longest to be shunted south. Model guidance has
trended downward with the magnitude of northerly winds over Lake
Michigan on Saturday and with it the potential for dangerous
swimming conditions at the beaches has also come down some. Can`t
rule out the northerly winds being strong enough to necessitate a
Beach Hazards Statement for portions of the lakeshore, but at this
time, a moderate swim risk and possibly hazardous conditions will
be messaged. High temperatures on Saturday will be in the lower to
mid 80s for most of the area away from the lake, with isolated
upper 80s in far interior sections, while upper 70s to around 80F
are expected near the lake. Humidity levels will start out
uncomfortable but will moderate with time (especially north of
I-80) as drier air works in from the north during the afternoon.
Castro
&&
.LONG TERM...
Issued at 203 PM CDT Fri Jul 28 2023
Saturday night through Friday...
Key Messages:
* Quiet weather through the weekend and majority of the week
* Temperatures return to seasonal average
* Chances for storms return towards the end of the week
Discussion:
A reprieve from the hot and stormy weather pattern will come this
weekend into next week, with temperatures returning to seasonal
average. The ridge will slowly weaken and push west. Expect highs to
be in the low-to-mid 80s through midweek next week, with cooler
temperatures along the lakeshore (potentially in the mid-to-upper
70s). Temperatures could push into the upper 80s towards the end of
the week. Dew points will drop through the weekend, retreating into
the 50s, bringing an end to the oppressive humidity. Dry air in the
mid-levels will limit the potential for rain and storms through
midweek. Chances for storms will return to the forecast late next
week.
Carothers
&&
.AVIATION...
For the 00Z TAFs...
Aviation Forecast Concerns:
* Period of scattered strong to severe thunderstorms this evening.
* Generally north-northeast winds through the period, though
variable wind directions in thunderstorms this evening.
* Low-confidence potential for a period MVFR ceilings late
tonight/early Saturday.
Early evening surface analysis depicts low pressure over northeast
IA, with with a nearly stationary frontal boundary extending into
southwest WI then southeast across northern IL where the effective
boundary appears to combine with the lake breeze off of Lake
Michigan. AMDAR aircraft soundings through the mid-late afternoon
have depicted a slowly weakening capping inversion near the 850 mb
level across he Chicago metro area which has resulted in fairly
slow growth of convective clouds along the lake breeze boundary,
though most recent soundings indicate this is eroding more quickly
now and suspect taller updrafts from west of KDPA southeast to
near KMDW and KGYY will soon be capable of lightning production.
Weaker capping upstream across parts of IA/southwest WI were being
overcome by large scale ascent associated with an eastward moving
mid-level disturbance, and it appears that convection will develop
more rapidly into our forecast area over the next couple of hours
as this forcing works its way east. Very warm/humid air mass
across the area combined with increasing mid-level flow aloft will
likely support the potential for strong to severe storms across
the terminals this evening, with damaging wind gusts and large
hail the primary severe threats. Storms likely to be in the
vicinity of several TAF sites at 00Z, with better coverage
expected to develop by/after 01Z. High-res CAM guidance shows some
differences in details of evolution and ending later tonight,
though it appears that storms may linger in the vicinity to/a
little past midnight.
Weather quiets down late tonight, with north-northeast winds
persisting through the remainder of the period. Winds may gust
near 20 kts at times by Saturday afternoon, especially within a
few miles of Lake Michigan. Some potential exists for a period of
MVFR ceilings into Saturday morning, with conditions otherwise
VFR.
Ratzer
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-
ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ103-ILZ104-
ILZ105-ILZ106-ILZ107-ILZ108 until 3 AM Saturday.
IN...Flood Watch...INZ001-INZ002-INZ010-INZ011 until 3 AM Saturday.
LM...None.
&&
$$
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Area Forecast Discussion
National Weather Service Phoenix AZ
514 PM MST Fri Jul 28 2023
.UPDATE...
Updated Aviation discussion.
&&
.SYNOPSIS...
Strong high pressure will continue to hover over the region with
Excessive Heat persisting into the weekend. Scattered afternoon
thunderstorm development will also continue over higher terrain
areas with the main impacts in lower elevation communities being
strong, gusty winds and blowing dust. However, an area of low
pressure will migrate northward out of Mexico over the weekend
bringing significantly increased chances of thunderstorms and cooler
temperatures to the entire area.
&&
.DISCUSSION...
Latest RAP analysis places the center of the Monsoon High along
the Arizona/New Mexico border. This is resulting in a light
southeasterly flow across central Arizona. Meanwhile, dewpoints in
the upper 50s and lower 60s are running a few degrees higher than
at this time yesterday. The high temperature has also eclipsed
the 110 degree mark in Phoenix for the 29th consecutive day.
Showers and thunderstorms developed early across northern Arizona
while satellite imagery reveals convection bubbling across the
White Mountains. The latest HREF suggests this activity will build
further south later today, particularly across Gila and Pinal
Counties.
Latest ACARS KPHX sounding depicts a layer of relatively dry air
near 800 mb, resulting in significant MLCIN across the lower
deserts, similar to yesterday. However, temperatures remain
relatively cool aloft, yielding steep mid-level lapse rates. There
is a 60 percent chance of an outflow boundary reaching the Valley
this evening from the northeast, but with abundant inhibition,
only isolated convection is possible along the boundary.
Nevertheless, there is a 30-50 percent chance the outflow boundary
will produce wind gusts of 35+ mph across central Arizona and
this is depicted well in the HRRR. There is an even higher
likelihood of gusty winds further south across Pinal County this
evening along with a 10 percent chance of severe wind gusts. Winds
of this magnitude will also be capable of producing areas of
blowing dust.
&&
.PREVIOUS DISCUSSION...
A well defined negative PV anomaly is already quite evident in WV
satellite imagery over eastern Mexico, and models continue to
propagate this feature westward into Sonora, and eventually up the
lower Colorado River valley over the weekend through early next
week. This classic late July inverted trough will enhance synoptic
scale ascent over the region resulting in markedly increased
thunderstorm chances and the likelihood of at least one organized
convective complex blasting through the region. The first vestiges
of this system will enter the picture Saturday with multi-model
output indicating a high probability (50-70% chance) of an intense
convective complex developing over northern Sonora/southern AZ. In
concert with other terrain forced storms and outflows, deeper
moisture should be thrust north and west into the forecast area with
the primary uncertainty as to whether downstream thermodynamic
profiles will be receptive to additional deep convection along and
behind the boundary. Given the probable thunderstorm intensity and
outflow depth, feel at least scattered storms are possible into
lower elevations Saturday evening with damaging winds and dense
blowing dust the main threats.
The most outstanding signal still hinges on Sunday as many favorable
conceptual aspects appear to be present for a complex of organized
storms in the CWA. Assuming the convective mass Saturday does not
fully contaminate and overturn the environment Sunday, deep boundary
layer mixing ratios of 10-11 g/kg should be available in a strongly
forced synoptic regime. Although model trends have shown the
aforementioned PV anomaly tracking a little farther south than
previously forecast and locally capping effective deep layer shear
around 30kt, a favorable coupling of MLCape/DCape around 1500 J/kg
still portends storms reaching strong to severe thresholds. Overall,
the pattern still looks to match some historical monsoon severe
events for the region.
Convective evolution beyond Sunday becomes more chaotic based on
mesoscale processes and the easterly wave weakening and becoming
stretched/sheared in southwest flow over the lower Colorado River.
Not surprisingly, the official NBM continues to spread relatively
high POPs over a large temporal and areal range Monday and Tuesday
not necessarily realistic of a monsoon pattern. Conceptually, the
focus for thunderstorm activity should shift into SE California and
NW Arizona Monday, then towards southern NV/UT Tuesday in advance of
the remnant circulation center. However with lingering moisture and
favorable ascent structure still over the region, cannot discount a
few storms anywhere in the CWA early next week. Thereafter, model
trends suggest a revision back to more limited deep convection
scenario as initially strong SW flow aloft acts to scour away better
moisture with mixing ratios perhaps falling back to an unsupportive
7-8 g/kg range. It`s possible during the latter half of the week
that strong subtropical ridging (598dm H5 heights) retrogrades back
over the forecast area further inhibiting thunderstorms, and also
reintroducing 110F temperatures into the forecast.
&&
.AVIATION...Updated at 0013Z.
South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Thunderstorms that are currently over Yavapai County and areas
further east/southeast are anticipated to have complex effects on
the surface winds over the Valley floor this evening. But, there
is a high degree of uncertainty on just how outflow driven wind
shifts will evolve. There is already outflow approaching far
northern portions of the Greater Phoenix area from the north and
northeast. One or two follow-on outflows from the east/southwest
are anticipated after that. Timing, strength, and precise
directions are all uncertain. The TAFs reflect the best estimate
of those parameters and subsequent updates are likely as things
unfold. Potential for new storm development over the Valley floor
itself (initiated by the outflows from distant storms) looks to be
limited but isolated storms are still possible. If any
development, follow-on wind shifts are likely. Apart from
outflows, surface winds will continue to favor westerly winds
through the evening (gusts 15-20kts early evening) and eventually
trend to light easterly before westerly breeziness resumes
Saturday evening. As for cloud cover, anticipate bases remaining
AOA FL120 (if not higher) except for the near vicinity of any
thunderstorms (dipping to FL090).
Southeast California/Southwest Arizona including KIPL and KBLH:
Thunderstorm activity is expected to remain far removed from the
TAF sites such that typical warm season wind patterns will
continue along with minor mid to high level cloudiness.
&&
.FIRE WEATHER...
Well above normal temperatures will continue into the weekend as
scattered afternoon thunderstorms develop over higher terrain
locations in the eastern districts. Wetting rainfall is likely with
a few of these storms, however a greater concern will be gusty and
erratic winds as well as the potential for new ignitions from
lightning strikes. Afternoon minimum humidity levels will continue
to fall into a 10-20% range following fair to good overnight
recovery of 25-50%. Periods of afternoon upslope gustiness will also
be common across all districts. A pattern shift with a marked
increase in moisture and thunderstorm chances will sweep into the
region over the weekend. Outside of the far western districts,
widespread wetting rainfall will be possible with potentially
repeated rounds of storms. This pattern shift will allow humidity
levels to moisten substantially while also tempering much of the
recent excessive heat. However by the middle of next week, drier and
warmer weather may return to most districts.
&&
.CLIMATE...
Record High Temperatures:
PHOENIX YUMA EL CENTRO
July 28 121 (1995) 124 (1995) 121 (1995)
July 29 115 (2020) 118 (1878) 115 (1954)
Active Streaks:
PHOENIX
1. Consecutive days of high temperatures 110F or greater: longest
on record at 28 days (previous record: 18 days in June 1974)
2. Consecutive days of no measurable precipitation: 4th longest on
record at 127 days
YUMA
1. Consecutive days of low temperatures 85F or greater: tied
3rd longest on record at 17 days
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Saturday for AZZ530-532-
534-537-538-540>544-546-548>551.
Heat Advisory until 8 PM MST this evening for AZZ557-558-563.
Excessive Heat Warning until 8 PM MST this evening for AZZ531-
533-535-536-539-545-547-552>556-559>562.
CA...Excessive Heat Warning until 8 PM PDT Saturday for CAZ562-563-
565>567-569-570.
Excessive Heat Warning until 8 PM PDT this evening for CAZ560-
561-564-568.
&&
$$
$$
DISCUSSION...Hirsch
PREVIOUS DISCUSSION...18
AVIATION...AJ
FIRE WEATHER...18
CLIMATE...Young