Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 191238 AAB

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
738 AM CDT Thu Apr 19 2018

.NEAR TERM...(Rest of Today)
Issued at 737 AM CDT Thu Apr 19 2018

Have made an early update due to cloud trends in southern TN into
northeast AL. Visible imagery depicts a broken to overcast deck has
arrived in our southern TN counties moving into Madison and Jackson
Counties. These will likely work their way a bit further south before
starting to erode through the day due to subsidence behind the upper
trough axis. Otherwise, no other significant changes were made to
today`s forecast.

.SHORT TERM...(Tonight through Saturday)
Issued at 415 AM CDT Thu Apr 19 2018

Benign and rather pleasant conditions can be expected in the short
term. Deep layer ridging establishing on the back side of the front
will stabilize the atmosphere and allow some advection of dry and
cool air. Only wave of unpleasantries will be the potential for some
patchy frost, mainly in S. Middle TN and low lying valley areas, on
Friday morning. With on-going cold air advection, temps are expected
to drop into the mid to upper 30s with weakening winds during the
morning hours. This will help produce some frost in those
aforementioned areas.

By Friday afternoon, low level ridging will be shifting NE helping
to shut off cold air advection. Temps will rebound back into the
upper 60s to around 70, and we`ll continue that warming trend into
Saturday with highs back around average (low/mid 70s). Given the
strong subsidence, no precip is expected through the period.

.LONG TERM...(Saturday night through Wednesday)
Issued at 445 AM  CDT Thu Apr 19 2018

Medium range NWP guidance remains in agreement this morning in
predicting a weakening cutoff upper low to advance east-
southeastward across southern OK on Saturday night. This feature is
then expected to shift eastward across AR on Sunday and into the mid-
South region on Sunday night, decelerating with time as it will be
blocked by an amplifying 500-mb ridge along the southeastern Atlantic
coast. Based on the weakening nature of the upper low, downstream
synoptic scale ascent will come primarily from increasingly diffluent
flow aloft, with this regime expected to begin early Sunday morning.
Although forecast soundings do depict a rather dry subcloud layer
early in the evening, upward vertical motions should result in fairly
rapid saturation from southwest-to-northeast prior to sunrise,
allowing light rain to slowly spread northeastward. This overall
trend will persist on Sunday and Sunday night, with lift being
enhanced during this period by strengthening lower- tropospheric WAA.
Rainfall is expected to be heaviest and most widespread from Sunday
afternoon through early Monday morning, when PWATS increase to
between 1-1.2 inches as the southeasterly low-level jet strengthens
to 40-50 knots.

Models suggest that the cutoff low will continue to weaken and may
drift very slowly east-northeastward on Monday-Monday night, as the
ridge axis along the Atlantic coast shifts further offshore. Although
the low-level jet may begin to slowly weaken across the western half
of the CWFA on Monday night, strong lift from low-level WAA should
still be in place to warrant fairly high POPs during the day, with a
more notable decrease in precip from west-to-east on Monday night.
Based on the anticipated coverage of clouds and precipitation, we
have advertised a muted diurnal temperature range from Saturday
night-Monday night, with lows in the l-m 50s and highs in the l-m

The cutoff low is forecast to degenerate into a trough and
accelerate east-northeastward from Tuesday-Wednesday, as it becomes
embedded within the flow around a stronger northern stream trough
digging southeastward across the northern Plains/upper MS Valley. The
low-level pressure gradient and wind field across the TN Valley will
relax as this occurs, and although POPs will be lower, a fair
coverage of low stratus clouds and perhaps some light drizzle will
remain across the region on Tuesday/Tuesday night. Mixing may
increase on Wednesday as a cold front associated with the northern
stream trough approaches from the northwest, and this could lead to a
bit more sun and slightly warmer temps.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 610 AM CDT Thu Apr 19 2018

VFR conditions will prevail today. Northwest winds will remain in
place through the afternoon hours and will gust to around 20kt
beginning ~18Z. Those northwesterly winds will gradually veer and
become more northerly after sunset and more northeasterly during the
overnight hours.





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