Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/09/23
Area Forecast Discussion
National Weather Service Denver/Boulder CO
735 PM MDT Tue Aug 8 2023
.UPDATE...
Issued at 716 PM MDT Tue Aug 8 2023
Severe thunderstorms have focused over the eastern plains this
afternoon and evening where abundant instability and bulk shear
have resulted in large hail up to 3" in diameter and even a couple
of tornados. ACARS soundings and SPC mesoanalysis indicates a
notable area of CIN over the urban corridor has been quite
persistent this afternoon, and prevented convective development
almost entirely. With some drier surface air filtering in as well,
opted to cancel the Severe Thunderstorm Watch at 7 pm for the
western counties that were in the watch. Still looks on track for
the eastern plains where a few more severe thunderstorms will be
possible through this evening as the shortwave axis moves
overhead. Otherwise, limited forecast changes this evening.
&&
.SHORT TERM...(This evening through Wednesday)
Issued at 300 PM MDT Tue Aug 8 2023
Key Messages:
1) Severe storms possible into the late evening hours
2) Destructive hail, high winds, and even a tornado or two
3) Still considerable uncertainty as to evolution/initiation of
storms.
Current satellite imagery shows the significant and atypically
strong shortwave trough moving east/southeast through southwest
Wyoming as of mid afternoon. This feature will continue on this
track taking essentially across southern Wyoming overnight. At the
surface, low pressure was developing in southeast Colorado, while
a cold front was pushing southeast east across southeast Wyoming
and northwest Colorado. There was a considerable cirrus shield
ahead of the shortwave, which is affecting destabilization and
convective development. That said, it now looks like we are seeing
enough destabilization to get a few storms going even through the
cirrus shield in spots. SPC surface analysis shows impressive MLCAPE
of 2000-2500+ J/kg across all of the northeast plains of Colorado,
with amounts dropping off closer to 500-1000 J/kg over the I-25
Corridor. ACARS soundings show weak CIN left along the Front
Range.
Despite the near erosion of CIN, there are still some
uncertainties with regard to development and extent of severe
convection due to the cirrus shield. It was clearly evident in
visible satellite imagery how the cirrus shield is affecting
cumuliform structure this afternoon. That said, it now looks like
we are seeing enough breaks to allow the threat of severe storms
to come to fruition.
There could still be two rounds of strong/severe convection - one
starting late this afternoon, and then another that develops as
the cold front, outflow boundaries, and upper level support/cold
pool aloft arrives. A 80+ knot jet is advertised to link up just
to our south by 03Z, putting northeast Colorado in the left exit
region of the upper level jet. This would weaken the cap and
support more numerous thunderstorm development, but there`s still
a fair amount of uncertainty of intensity due to the conditional
nature of available instability. And that would be based on how
much if any CAPE would be used up by the first round (if it fully
develops)! In summary, all ingredients are there for severe
convection and potential for destructive hail, high winds, and
even a couple tornadoes over the northeast plains. The highest
threat area will be roughly along/east of a line from Kimball,
Nebraska to Fort Morgan and Limon. There is a much lower, but
non-zero threat of less severity of storms farther west toward
the I-25 Corridor. Meanwhile, mountain areas will see scattered
less intense showers and a few storms through the midnight hour.
Finally, the second batch of storms could produce locally heavy
rain and some flooding concerns due to training over the far east
central plains late this evening.
The storms will linger into the late night hours tonight thanks to
the upper level support and trough passage. By daybreak, the
lingering showers and storms will be pushing east into Nebraska
and Kansas, with mostly sunny skies dominating the forecast area
Wednesday. There is some cold advection occurring tonight, so
tomorrow`s highs will be several degrees below normal (lower to
mid 80s over the plains). No showers or storms are expected as a
subsident and capped airmass will dominate.
.LONG TERM...(Wednesday night through Tuesday)
Issued at 300 PM MDT Tue Aug 8 2023
Wednesday night will see the continuation of our one dry day/night
across Northern Colorado under a subsident and dry westerly flow
aloft. Thursday will see a continuation of the westerly flow aloft
but some slight increase in moisture seeping in from the southwest.
This may result in some increase in high based showers/storms in
the mountains but doubt there is much development across lower
elevations with very little instability and dry low levels. Temperatures
will also be warmer as thermal ridge rebuilds over Colorado with
700mb temperatures back up around +16c.
From Friday and especially this weekend expect a slow increase in
monsoonal moisture from the desert southwest given an upper low
over California and high pressure ridge over Texas. The combination
of these features will stream deeper moisture back into Colorado
this weekend as precipitable water values closing in towards an
inch by late Saturday evening and Sunday. Forecast soundings
showing only marginal MLCAPE with mid level flow decreasing. This
will bring more of a flash flood threat and lesser severe weather
threat with these factors through early next week. Temperatures
will be back to near normal through the period.
&&
.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 528 PM MDT Tue Aug 8 2023
Becoming increasingly confident in thunderstorm activity remain
east of the I-25 corridor and KDEN this afternoon and evening.
Cannot completely rule out a brief shower, primarily in the 03-06Z
timeframe, but the potential for strong thunderstorms is
diminishing.
A surface boundary will move through the Denver area in the next
1-2 hours and sustain northerly winds through the evening, before
a return to lighter drainage flow overnight.
Tomorrow, expect VFR conditions through the day with no
thunderstorm activity expected. Winds will follow typical diurnal
patterns with a NE component in the afternoon and drainage in the
evening/overnight.
&&
.HYDROLOGY...
Issued at 300 PM MDT Tue Aug 8 2023
There is some risk of flash flooding over the northeast and east
central plains of Colorado, due to storm intensity and potential
training of storms. There will be upward forcing for greater storm
coverage, and given high precipitable water values and significant
low level moisture (dewpoints in the lower 60s), stronger storms
will be producing heavy rain. Most storms will produce 1-2 inches
of rain in an hour, while training storms could dump as much as
3-4 inches and produce local flooding issues.
For the burn areas, the flash flood threat is limited (but non-
zero) due to faster storm motion.
Wednesday will be dry so there is no threat of flooding.
Some mid level moisture will arrive Thursday and Friday which
will create isolated storms. These storms will not be particularly
strong nor will they have ample moisture so the burn area flash
flood threat will be limited.
Above normal monsoonal moisture will arrive this weekend and into
Monday. Coverage and strength of storms will increase and they will
be slow-moving so these days may end up seeing a burn area flash
flood threat closer to elevated.
&&
.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
UPDATE...Rodriguez
SHORT TERM...Barjenbruch
LONG TERM...Entrekin
AVIATION...Rodriguez
HYDROLOGY...Barjenbruch
Area Forecast Discussion
National Weather Service Phoenix AZ
405 PM MST Tue Aug 8 2023
.UPDATE...
Updated Aviation
&&
.SYNOPSIS...
Monsoonal moisture will lead to continued rain chances of 10 to 30
percent on the lower deserts and 30 to 50 percent over the higher
terrain through Thursday. A gradual decline will take place from
west to east beginning Friday with storm chances limited to south-
central Arizona. Anticipate high temperatures to be lower than they
have been in several weeks but still near, to slightly above,
normal. A warming trend is anticipated for early next week with
highs climbing above 110 at most lower desert locations by next
Tuesday.
&&
.DISCUSSION...
Deep southwesterly flow remains in place which is aiding coherent
storm movement to the northeast. The development areas have been
over higher terrain locales of the eastern third of AZ plus Pima
County west of Tucson. With the storm movement to the northeast, it
will be hard to get storms to advect to our forecast area. However,
outflow from Pima County storms could aid propagation to the north.
Latest aircraft soundings indicate there isn`t a lot of CIN over the
Phoenix area. We may wind up having something of a repeat of last
night/this morning with a passing disturbance tapping into the
convective potential. This time, the models are picking up on a weak
short wave emanating from the upper trough where the remnant
circulation of former TS Eugene is. There is also a moisture
convergence boundary over the Mohave Desert that is over/near our
CWA border with VEF CWA. That boundary is anticipated to nudge
further north and may be another aid to convection. Most of the
deterministic CAMs are depicting relatively weak echoes/showers late
tonight - mainly over VEF and FGZ CWAs. The HRRR has had run- to-run
variations in showing very isolated storms over La Paz County with
better coverage just north. The NBM PoPs once again has a late
night/morning peak over the whole forecast area (except for portions
of Gila County which peak this afternoon/eve). That being said, the
numbers aren`t overwhelmingly high but definitely higher than
seasonal climatology. Though not convection related, model data
shows strong winds for late afternoon and tonight over western
portions of Imperial County extending into the Imperial Valley.
Thus, a Wind Advisory was issued.
The convective outlook for later Wednesday has a lot of uncertainty.
The moisture availability will be relatively good. But, being on the
back side of the disturbance, there may be some subsidence to hinder
things. Also, where/if there is prior precip, that will hinder
destabilization as well. But, if the preceding wave isn`t much, then
potential will be better. Global models are depicting a gradual
downtrend in moisture availability beginning Thursday but not a
dramatic drop off. The NBM PoPs reflect this with an eastward
retreat - most noticeable on Friday - but lingering over south-
central AZ through next Tuesday.
Temperatures are expected to be within a few degrees of normal for
the next several days thanks to the influence of troughing and
eastward retreat of the strongest high pressure. But, that still
means moderate HeatRisk as opposed to no issues at all. Early next
week, the ridge slowly expands westward again leading to an uptrend
in temperatures - potentially back to excessive thresholds by next
Tuesday. Of note, the NBM deterministic value for next Tuesday is at
the 75th percentile.
&&
.AVIATION...Updated at 2305Z.
South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major weather impacts are expected though Wednesday afternoon
under occasional mid/high cloud decks. Confidence is only moderate
with respect to timing overnight wind shifts as most evidence shows
a somewhat earlier shift to easterly versus the past several days.
Chances for outflows and any storm development affecting terminals
are too low to mention in this TAF package. Otherwise, west winds
with a few higher gusts should be common again Wednesday afternoon.
Southeast California/Southwest Arizona including KIPL and KBLH:
Gusty winds, particularly at KIPL will be the greatest weather issue
under periods of mid/high cloud decks. Winds will tend to stay
westerly at KIPL through Wednesday afternoon with gusts ~30kt common
this evening resulting in some lofted dust. Directions at KBLH will
vary more between SE and SW with gusts more muted 20-25kt.
&&
.FIRE WEATHER...
Monsoonal moisture will linger through Thursday for ongoing, though
modest, potential for thunderstorms at any given location. High
temperatures will be at, or a few degrees above, normal. MinRH
values will be in the 20-30% range and MaxRH values in the 40-60%
range most places. Eastern Riverside County will see the most
significant increases in humidity. Late in the week and over the
weekend, anticipate a gradual decline of humidity as well as storm
chances. Temperatures trend upward early next week to well above
normal. Apart from thunderstorms, winds will favor familiar warm
season patterns - except over Imperial County with late afternoon
and nighttime windiness from the west today and Wednesday.
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...Wind Advisory from 5 PM this afternoon to 8 AM PDT Wednesday for
CAZ562.
Wind Advisory from 5 PM this afternoon to 2 AM PDT Wednesday for
CAZ563-566-567.
&&
$$
DISCUSSION...AJ
AVIATION...18
FIRE WEATHER...AJ