April 2021

General Weather Discussions and Analysis
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DoctorMu
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Re: April 2021

Post by DoctorMu » Mon Apr 19, 2021 2:25 pm

don wrote:
Mon Apr 19, 2021 10:46 am
Models are showing possibly multiple rounds of heavy rain and severe thunderstorms Friday and Saturday. As the frontal boundary briefly stalls which allows multiple disturbances to ride the front over the area.
Big Friday afternoon and evening event for us. All about timing.

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jasons2k
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Re: April 2021

Post by jasons2k » Mon Apr 19, 2021 3:20 pm

Great discussion:
Area Forecast Discussion
National Weather Service Houston/Galveston TX
340 PM CDT Mon Apr 19 2021

.DISCUSSION...

.SHORT TERM [Through Tuesday Night]...

The quiet weather pattern continues today thanks to surface high
pressure sitting over Southeast Texas. As the high slides off to
the east, winds will transition from easterly to southeasterly.
Temperatures today will be slightly below normal with highs in the
low-to-mid 70s. For tonight, temperatures will be a bit warmer
than the previous night since CAA has come to an end. Overcast
clouds moving in overnight will initially keep temperatures on the
mild side, but these will depart after midnight allowing
temperatures to drop into the low 50s. It`s worth mentioning that
some hi-res models are indicating isolated rain showers developing
after sunset due to upper-level divergence from a departing jet
streak, but with plentiful dry air in the lowest 3km above the
surface, any precipitation that falls will evaporate before it
reaches the ground. Thus, PoPs will remain less than 10%
throughout the night.

Southwesterly winds kick in and skies clear out on Tuesday morning
as surface high pressure continues its eastward trek. Resultingly,
afternoon temperatures will be slightly warmer with highs
approaching the 80 degree mark at most locations. The southwesterly
winds will gradually decrease our PW values down into the 0.6 inch
to 0.8 inch range, so our moisture profile looks relatively slim
ahead of our next cold front that will pass through the region on
Tuesday afternoon/evening. Since there will be dry air out ahead
of the front, models are in agreement that this will be a dry
FROPA. With sufficient CAA persisting through Tuesday night,
temperatures will be at least 10-15 degrees below normal with lows
ranging from the low 40s to the low 50s across Southeast Texas.

.LONG TERM [Wednesday Through Monday]...

Wednesday morning puts us in a decidedly post-frontal environment,
as a surface high sits over the Red River Valley. The cold
advection does not look particularly strong, but we also weren`t
starting from the warmest, most humid place either, so Wednesday
still looks to be unseasonably cool and dry. Of course, we are
now arriving in the time of year where your reaction to that
sentence depends wholly on how you feel about heat and humidity.
For this Wisconsin-raised forecaster, a sunny day with highs in
the upper 60s to 70s with dewpoints ranging from the upper 30s to
lower 50s sounds downright wonderful! But your mileage may vary.

Spring being a season of transition, this setup won`t stay static,
particularly since the flow aloft is relatively zonal. Wednesday
night will see winds veer to become more easterly, picking up a
connection with more humid Gulf air. Meanwhile, a surface low
should be starting to spin up over New Mexico underneath what
looks like it will be a pretty robust shortwave trough dropping
off the Rockies. This will keep the pressure gradient fairly
tight, for the back half of the week, aiding the return of deep
moisture over the area.

Depending on how quick moisture return is, we could see some low
chances for showers as early as late Thursday afternoon down by
Matagorda Bay closer to the deep moisture axis. However, by Friday
morning, the potential for showers will spread across our area.
Depending on how significant capping is, we could see some
afternoon convection on Friday, particularly if we see some pre-
frontal surface troughing as a focus mechanism, as hinted at in
at least some of the guidance. The other focus will come later
Friday night into Saturday as the front sweeps through. Though the
timing is not best, there looks to be enough instability that we
could sustain some existing, organized severe weather for some
time right on the front. SPC does have the western half or so of
our area in what is the equivalent of a slight risk area for
Friday night, and that seems a reasonable forecast. For what is a
Day 5 forecast, there`s only so much that can be divined from what
will eventually be driven by mesoscale details.

Speaking of mesoscale details, the GFS has a party with them...or
at least the best approximations of them it can resolve with the
frontal convection. There`s really zero point in trying to
speculate on details or specific timing, but it`s probably worth
being on the lookout for some kinks in the forecast as the front
rolls through.

Peering into early next week, there doesn`t appear to be much cold
advection behind the front to speak of, so this may well be a
situation where the increasingly powerful sun overwhelms any
potential cooling behind the front, and we actually end up with
warmer temperatures after the front passes. Indeed, that`s just
what I have in the forecast. My sweet, spring children, summer is
coming.

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jasons2k
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Re: April 2021

Post by jasons2k » Mon Apr 19, 2021 10:14 pm

NWS Click Forecast:

Friday:
A chance of showers and thunderstorms, then showers and possibly a thunderstorm after 1pm. High near 77.

Chance of precipitation is 80%.

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DoctorMu
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Re: April 2021

Post by DoctorMu » Tue Apr 20, 2021 12:55 am

Friday looking less impressive on 00Z GFS and CMC.

Cromagnum
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Re: April 2021

Post by Cromagnum » Tue Apr 20, 2021 7:07 am

DoctorMu wrote:
Tue Apr 20, 2021 12:55 am
Friday looking less impressive on 00Z GFS and CMC.
Lucy been getting in a ton of practice lately.

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Re: April 2021

Post by Cpv17 » Tue Apr 20, 2021 7:14 am

It looks like we will still have a severe weather threat Friday evening and we could still get 1-2” for a lot of southeast Texas but the best chance for any significant rain will be north and east of us. Looks like Louisiana into Arkansas and Mississippi will get the bulk of the rain.

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don
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Re: April 2021

Post by don » Tue Apr 20, 2021 9:22 am

This is looking to be more of a severe weather event more than flooding.Saying that, street flooding especially in urban areas is a possibility. It seems that models have come into agreement with the main surface low being further inland in the panhandle region. This decreases the amount of rainfall potential locally, but at the same time the surface low being further north should produce a more substantial warm sector,increasing the chance of severe weather here. SPC mentions that all modes of severe weather may be possible Friday.


day48prob.gif
Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0358 AM CDT Tue Apr 20 2021

Valid 231200Z - 281200Z

...DISCUSSION...
Low-level moisture return appears likely across parts of the
southern Plains on Day 4/Friday as lee cyclogenesis occurs across
the southern High Plains ahead of an ejecting upper trough.
Medium-range guidance is now in reasonably good agreement that this
shortwave trough will eject eastward over the southern Plains on
Friday. Increasing low-level moisture to the east of a dryline/cold
front and steep mid-level lapse rates should support at least
moderate instability in a corridor across parts of OK and TX by
Friday afternoon. Strong mid-level flow and related deep-layer shear
appear favorable for organized severe storms, including supercells.
The low-level moisture should be somewhat greater in TX compared to
OK. As mid-level height falls/ascent overspread the warm sector,
robust convective development appears likely, with at least isolated
severe storms posing a threat for all severe hazards potentially
spreading eastward towards east/southeast TX and parts of the lower
MS Valley Friday evening/night in a strong low-level warm advection
regime.

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Re: April 2021

Post by Cromagnum » Tue Apr 20, 2021 9:37 am

Figures. Every year it happens where we go through a dry period so all the fence posts in the yard get loose with the ground cracking. Then we don't get rains to stiffen things up again, we get storms to blow them down.

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DoctorMu
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Re: April 2021

Post by DoctorMu » Tue Apr 20, 2021 9:58 am

Cromagnum wrote:
Tue Apr 20, 2021 9:37 am
Figures. Every year it happens where we go through a dry period so all the fence posts in the yard get loose with the ground cracking. Then we don't get rains to stiffen things up again, we get storms to blow them down.
haha That's one thing I learned about fenceposts in the Brazos Valley. Steel posts with hardware and an 80 lb bag of cement + half a bucket of water. The fence may rot away but the posts remain!

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don
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Re: April 2021

Post by don » Tue Apr 20, 2021 3:00 pm

Area Forecast Discussion
National Weather Service Houston/Galveston TX
317 PM CDT Tue Apr 20 2021


.LONG TERM [Wednesday Night Through Tuesday]...

Surface high pressure slides off to the east going into Wednesday
night bringing back a southeasterly flow to the region. Still
expecting another night of below normal temperatures though with
lows in the upper 40s/low 50s. With the onshore flow comes an
increase in moisture and an increase in temperatures! MaxT values
will reach the low-to-mid 70s on Thursday and the upper 70s on
Friday, the latter of which is seasonal for this time of year. With
steady WAA, PW values increasing to 1.2"-1.4" by Thursday afternoon,
and upper-level divergence, scattered rain showers are expected to
develop initially in the western portion of the CWA and spreading
eastward. The big story, which I`m sure you all are aware of, is
coming up for Friday/Saturday.

A shortwave trough out ahead of an upper-level trough will generate
lee cyclogenesis. Developing surface low pressure in north TX will
increase low-level moisture return with PW values increasing to
nearly 2" by Friday afternoon. Resultingly, widespread showers and
thunderstorms are expected Friday afternoon/Saturday morning. SPC
does have the entire CWA outlined in a 15% chance of severe weather
for Friday. With a strong inflow of low-level moisture and steep mid-
level lapse rates supporting sufficient instability, I don`t have
any reason to disagree with them. The probability of strong to
severe storms will exist ahead of (Friday afternoon) and along the
boundary of a "cold front" (Friday night/Saturday morning). This is
still three days out, so it`s too early to set things in stone. The
picture will become clearer once we get into the range of hi-res
model/CAM guidance.

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