Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 10/06/21

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 902 PM CDT Tue Oct 5 2021

.UPDATE...
902 PM CDT

Main change to going forecast was to adjust sky cover grids for
more clearing this evening. Clearing line continues to slowly
march northwest across the CWA and now looking plausible the
clearing could even reach Rockford and west of I-39 corridor this
evening. A couple of recent ACARS soundings into ORD show that the
low levels remain very moist and it would take very little cooling
to reach saturation. Thinking that there will be enough mixing to
prevent dense fog from developing, but given the lengthening
nights, unseasonably moist airmass, and already small T/Td
spreads up to 900mb that stratus will re-develop tonight. If (and
where) stratus develops, wouldn`t expect quick clearing tomorrow
morning and this could have implications (possibly big ones) for
tomorrow`s forecast high temperatures. Didn`t make any adjustments
to tomorrow`s temps with this evening update as a lot will hinge
on the development of stratus. If stratus fails to materialize,
then tomorrow`s forecast highs would be on target, however if
stratus develops and lingers as it is looking like it might, then
temps may struggle to get out of the 60s (similar to today).
Updated grids and derived products have been sent.

- Izzi

&&

.SHORT TERM...
228 PM CDT

Through Wednesday night...

Mostly cloudy but seasonably mild weather looks to persist through
mid-week, with a return of better rain chances especially by later
Wednesday and Wednesday night.

Early afternoon water vapor imagery depicts an elongated upper
trough stretching from New England, across the Midwest and lower
Great Lakes region to a closed upper low over the lower Mississippi
Valley. North of this upper trough axis, relatively strong mid-level
ridging (5890 meters at H5) was building across the upper
Mississippi Valley and northern Lakes. A weak short wave was also
evident tracking westward across central IL/IN, though mid-level
dryness doesn`t depict much in the way of deeper cloud features with
this disturbance. Radar does depict some shallow, isolated/widely
scattered light showers generally south of the I-80 corridor
associated with the wave. Passage of the wave off to our west,
combined with a gradual diurnal loss of any marginal/weak low-level
instability should allow these to end by evening. Perhaps the
toughest forecast through tonight will be in terms of cloud cover.
While there is not much in the way of drier low level air upstream
to our northeast, subsidence across WI and lower MI associated with
the building upper ridge (part of a REX-block pattern with the
closed low to our south) may allow for some breaks in the lower
overcast which could advect into our area overnight. Guidance
generally depicts low clouds persisting, though would not be too
surprised to end up with some breaks in some spots. This sets up the
potential for low temp busts tonight, where any clearing would
likely allow temps to cool farther into the 50s. With surface dew
points still holding near 60 this would also potentially lead to
patchy fog development overnight as well. Parts of northwest IN and
east central IL might be most likely to see this, where low level
winds are not off the lake. Otherwise, where clouds linger lows in
the upper 50s to around 60 are expected.

By Wednesday, attention turns to the upper low off to our south,
which is progged lift slowly northward toward the area as the upper
ridge to our north/northeast moves east and the REX block weakens.
Scattered showers on the northern periphery of the circulation will
gradually spread into the area from the south from late morning into
the afternoon, with the best chances remaining generally south of I-
80. Increasing isentropic ascent and moisture transport into
Wednesday night should spread showers and some thunderstorms north
across the remainder of the forecast area overnight. Moist column
with Pwats above 1.50" and deep warm cloud depths may again support
some locally heavier rainfall amounts with these showers, though the
best coverage and greater QPF amounts appear to come in the start of
the long-term forecast on Thursday.

Temperature wise, we`ll remain milder than average for early October
in this pattern. After highs in the mid-60s to low 70s today, low to
mid 70s appear likely Wednesday with a few upper 70s possible if
cloud cover is a little thinner in spots. Lows will be in the upper
50s and lower 60s for the most part, as opposed to the 40s/low 50s
more typical of this time of year.

Ratzer

&&

.LONG TERM...
245 PM CDT

Thursday through Tuesday...

As noted above, the upper low will continue to lift north toward
the local the area on Thursday. This will provide upper level
support for lift and in tandem, low-level moisture will feed into
the area out of the southeast. There is some slight timing
differences with when the center of the low moves overhead, but in
general, expect increasing coverage of showers across the area
into Thursday afternoon with some embedded thunderstorms also
possible. Precip chances will continue overnight into Friday with
spotty showers possible on the back side of the departing low.
Temperatures, while still unseasonably warm, will remain mainly in
the lower to mid 70s.

Upper ridging builds in behind the system Friday night into the
weekend with temperatures expected to warm well above normal into
the upper 70s and lower 80s Saturday and Sunday. A system well to
our north will push a cold front into the Great Lakes region on
Sunday. In advance of this front expect gusty winds out of the
south and southwest which should also help temps warm into the
80s on Sunday. This should keep temperatures unseasonably warm and
humid, even near the lake front, something to keep in mind for
strenuous activities outdoors.

Beyond the weekend, the western upper trough begins to eject
across the Central Plains and will bring us additional chances for
precipitation early next week.

Petr

&&

.AVIATION...
For the 00Z TAFs...

Lower than average forecast confidence with respect to CIGS and
VSBY tonight into Wednesday with forecast model guidance not
handling current trends all that well.

Satellite imagery shows clearing spreading northwest into the
Chicago terminals early this evening, a development not well
handled by current suite of forecast model guidance. Anticipate
this clearing to continue to spread northwest, probably slowing
some early this evening now that the sun has set. A period of VFR
conditions is likely this evening at MDW and GYY, and probable at
ORD and DPA.

Atmosphere in the low levels remains unseasonably moist and any
clearing should allow for the development of fog or low stratus
tonight. At this point, thinking is that there will be enough wind
to keep fog from forming with the more likely scenario being IFR
stratus developing following the clearing/partial clearing this
evening. Where MVFR CIGS linger, like RFD, some build down to IFR
is possible, but confidence is low in when/if that will take
place. If winds diminish more than forecast, then the threat of
dense fog would increase substantially, particularly at DPA/GYY,
but threat of dense fog early Wednesday morning at ORD and MDW
(while low) is not zero.

Where ever CIGS start Wednesday morning, expect improvement to be
slow to occur with continued lowering sun angle and low levels of
atmosphere remaining so moist. Some showers could spread north and
potentially impact the terminals very late Wednesday afternoon,
but the better chance looks to be Wednesday evening.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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Area Forecast Discussion

National Weather Service Los Angeles/Oxnard CA

 828 PM PDT Tue Oct 5 2021

.SYNOPSIS...05/828 PM.

A cooling trend is expected through Friday with an increase in
night through morning low clouds. A frontal system may bring rain
to the entire area by Thursday night and Friday. Dry and warmer
weather is expected for the weekend but a cold upper low will
bring cooler temperatures and windy conditions early next week.

&&

.SHORT TERM (TUE-FRI)...05/827 PM.

***UPDATE***

Marine layer is making its return known this evening. Solid low
clouds are pushing into the Central Coast like it usually does.
Over areas to the south however, coverage is mush more sporadic.
Expecting the coverage to solidify more over the next few hours,
as the lasted aircraft soundings out of LAX shows a more developed
marine layer than previously expected. The marine layer is shallow
however, currently at or below 1000 feet. That should deepen over
Los Angeles and Ventura Counties tonight, but should stay shallow
elsewhere. The current forecast is in good shape with no need to
update.

***From Previous Discussion***

In any case, tomorrow is expected to be cooler as well with most
areas down 2-5 degrees from today.

Any marine layer clouds should mostly clear off by late morning
but high clouds will be increasing through the day from an
approaching trough from the northwest. So just a partly cloudy day
at best tomorrow.

Additional marine layer deepening expected Thursday as the trough
gets closer and heights drop. Would not be surprised if there is
some morning drizzle, especially south of Pt Conception. Marine
layer stratus expected to push up the coastal slopes with minimal
clearing expected and daytime temperatures mostly in the 60s at
lower elevations.

The trough will move onshore Friday morning with the best combo of
PVA and moisture south of Pt Conception. Pops are weighted
accordingly, with 60 or higher in LA County down to 40 or less
across San Luis Obispo County. Amounts expected to favor
southeastern LA County with the best upslope enhancement. Still,
overall amounts expected to be a quarter inch or less, especially
if the GFS is correct as only a few of the its ensemble members
show anything. The ECMWF has been pretty consistent showing a
better tap of moisture to the southwest. If such a tap happens the
higher end of amounts could be a half inch in upslope areas of LA
County or locally higher.

.LONG TERM (SAT-TUE)...05/140 PM.

Following the Friday system light northeast flow will develop over
the weekend with a little ridge overhead. Pressure gradients
actually turn lightly offshore Sunday to accompany the northeast
flow aloft resulting in clear skies and a significant warming
trend with Sunday being the warmest day of the next several. Highs
in the mid to high 70s for most coastal zones and close to 90s in
the warmest valleys.

That will all come to a crashing halt Monday and Tuesday as a cold
upper trough drops south along the West Coast. The trajectory is
not favorable for moisture so it`s not expected to be a big rain
maker, though an isolated shower or two can`t be ruled out by
Monday night into Tuesday, especially north facing slopes and LA
County. And it`s cold enough that if there is enough moisture it
could fall as snow over the interior mountains. However, much
more impactful will be the gusty north to northwest winds that
develop late Monday into Tuesday, likely advisory level at least
for southern Santa Barbara County and the I5 corridor with gusts
to 50-60 mph possible and gusty winds many other areas as well.

&&

.AVIATION...06/0111Z.

At 23Z over KLAX, there was no marine layer or inversion.

Moderate confidence in presence of low clouds tonight. Low
confidence in timing, with plus or minus 5 hours from TAFs
possible. Moderate confidence of IFR/LIFR CIGS being most common
for KSBP KSMX KSBA. Low confidence in CIG cats elsewhere, with a
chance of IFR especially through 12Z, with MVFR OVC015-025 more
likely after 12Z. KPRB currently expected to be CIG free, but has
a 30% chance of IFR CIGS after 10Z. High clouds streaming will
block any view of low clouds from satellite tonight for KPRB KSBP
KSMX KSBA, making it hard to monitor.

KLAX...Low confidence on evolution of low clouds tonight. 80%
chance of CIGS sometime tonight, starting anytime between 03Z and
09Z. Moderate confidence of MVFR CIGS after 15Z Wed. Low
confidence before, with CIGS between BKN005 and BKN015 possible.
No significant east winds expected.

KBUR...Moderate confidence in VFR through at least 06Z. OVC008-15
starting 08Z-12Z.

&&

.MARINE...05/800 PM.

High confidence in the current forecast through Wednesday, then
moderate confidence exists thereafter. Winds and seas are
expected to remain below Small Craft Advisory (SCA) levels for
all of the coastal waters through Wednesday morning. Winds are
expected to increase to SCA speeds in zone 670 and perhaps the
northern portion of zone 673 Wednesday afternoon and evening.
Winds will increase again to SCA levels Thu night over the outer
waters, and there is a 50% chance of these conditions Friday
through Sunday, including the inner waters N of Point Sal.

Inside the southern California bight, there is a moderate chance
of SCA level winds for the east Santa Barbara Channel Friday
through Sunday.

A large northwest swell could possibly build in early next week
along with strong gales and steep seas across much of the outer
waters.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.

&&

.HAZARD POTENTIAL OUTLOOK (FRI-TUE).

Widespread moderate to strong northwest to north winds expected
Sunday night through Tuesday, including southern Santa Barbara
County, the I-5 Corridor, and most coastal waters. Downed trees,
power outages, large surf with strong currents, and dangerous
driving and boating conditions expected.

&&

$$

PUBLIC...MW/RK
AVIATION...Kittell
MARINE...Sirard
SYNOPSIS...MW

weather.gov/losangeles



Area Forecast Discussion

National Weather Service Phoenix AZ

 458 PM MST Tue Oct 5 2021

.UPDATE...
Updated Aviation discussion.

&&


.SYNOPSIS...
A low pressure system will continue to move through the Southwest
today. Periods of showers and thunderstorms will continue this
morning, then gradually end from west to east this afternoon and
evening. Severe thunderstorms are expected to develop this
afternoon, capable of producing strong, damaging winds, large
hail, and perhaps an isolated tornado. Some blowing dust will
also be possible. Dry weather returns tomorrow and Thursday with
near normal temperatures. A low end rain chance is possible with
another passing system Friday and Saturday with slightly cooler
temperatures. A quiet weekend with typical temperatures is expect,
with the potential for a dry but notably cooler and winder
weather system next week.

&&

DISCUSSION...
A compact upper level low is moving east near the
California-Mexico border. East of this low, warm and moist air
continues to stream northward into the forecast area, leading to
scattered showers and thunderstorms. So far, the relatively dry
and cool boundary layer has only allowed for elevated convection.
Rainfall and wind have both been limited, but we have received
several reports of small hail. As we progress into the afternoon,
several factors will come together for a potentially significant
severe weather event by Arizona standards.

Visible satellite imagery continues to show mostly clear
conditions south of Phoenix, with a mixture of clouds and sun over
the metro area. Overall, this is allowing for good low level
heating. The 18z PSR sounding and ACARS data confirm this to be
the case, with a gradually warming boundary layer. Model
forecasts show that this should allow for surface temperatures to
warm enough to overcome any convective inhibition by this
afternoon. As convection becomes increasingly tied to the surface
layer, it will dip into the strong low level directional shear
present today. Meanwhile, as the upper level low advances towards
the area (along with a corresponding jet streak) upper level wind
speeds will increase along with diffluence aloft. This will result
in effective bulk shear values of 40-50 kts continuing into the
early evening. In addition to the favorable wind shear, upper
level temperatures will cool significantly with 500 mb
temperatures dropping to -15 to -16C or so. Very steep lapse rates
into the mid levels will allow for a favorable hail growth zone
to develop. CAPE values of 1500-2000 J are anticipated by the mid
afternoon. Lastly, PWAT values will remain in the 1-1.2" range,
which is easily sufficient for convective development.

Taken together, the environment will be well primed for supercells
to develop this afternoon with large hail (potentially up to 2"
or so) and high wind gusts (potentially up to 65 kts or so). The
strong low level shear may also allow for an isolated tornado or
two. the most favored location for tornadic development is likely
north of the Phoenix metro. The SPC has issued a severe
thunderstorm watch through 03Z this evening to account for these
threats.

The risk for flash flooding appears to be much less than the risk
for severe weather due to fast storm motions. Even so, as stronger
thunderstorms develop this afternoon, the potential for locally
heavy rain will increase. Accordingly, an isolated flash flood
warning cannot be ruled out today, especially over the burn scars.

The HREF has handled the strongly forced convective pattern quite
well to this point. It indicates that scattered showers and
storms will continue throughout the day, but that the strong to
severe storms are likely to develop roughly along a line from west
of Phoenix up through the Prescott area around 1-4 pm this
afternoon. These storms will then likely traverse through the area
between 3-7 pm. The threat of precipitation in the PSR forecast
area will quickly wind down after that. As always, the exact
locations which will see severe weather is never known for sure
ahead of time and conditions can change quickly. Today would be a
good day to check back frequently for forecast updates.

After today`s active weather, the next few days will be quiet with
highs near 90 under partly sunny skies. Global ensembles indicate
that another system will graze the area Friday night into
Saturday. The chance of precipitation is quite low, with NBM
showing less than a 10% chance of measurable precipitation.
Temperatures will likely cool off though, with highs in the mid
80s over the weekend. An even stronger storm system (also likely
dry) will affect the area next week. Details are limited at this
range, but there is excellent agreement that temperatures will
cool dramatically. The NBM only shows a 55% chance of exceeding
72F next Wednesday in Phoenix. Should this occur, morning lows
would fall into the 40s and 50s for the first time since last
spring.

&&

.AVIATION...Updated at 2357Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Another round of thunderstorms is expected to affect the metro
area roughly between 00Z-03Z. Anticipate that western portions of
the Valley will see the brunt of storms. But, it is expected to
cause a major wind shift with a westerly component at KPHX
between 01Z- 02Z and be close enough for VCTS but otherwise not
lower ceilings below 6kft AGL. But, there is definitely a
possibility that KPHX could have TSRA+ at the airfield which would
cause even more erratic winds and a major drop in ceilings and
visibility along with hail potential. Anticipate the Valley will
mostly be clear of thunderstorms by 04Z if not 03Z. Apart from
thunderstorms, anticipate northeasterly surface winds will
transition to northerly this evening then transition to familiar
nocturnal patterns near midnight. Very benign conditions are
anticipated during the overnight hours and daytime Wednesday.

Southeast California/Southwest Arizona including KIPL and KBLH:

Shower and storm activity has ended over southeast California and
southwest AZ. There are some lingering batches of cumulus/altocumulus
which should thin out after sunset. Surface winds at KIPL will
favor familiar diurnal patterns at KIPL. At KBLH, anticipate
northeasterly winds will transition to northerly late tonight with
a period of light and variable in the morning before light south
and southeast develops in the afternoon. Otherwise, no significant
cloudiness.

&&

.FIRE WEATHER...
Thursday through Monday:
One weather system will move through the region Thursday and
Friday, bringing low end rain chances and a slight uptick in
afternoon humidity. Slightly cooler and drier weather arrives
Saturday, but warmer and drier weather is expected Sunday and
Monday. Overnight humidity values will generally be trending down
through the entire period.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...Hodges
AVIATION...AJ
FIRE WEATHER...Iniguez