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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 250203

Area Forecast Discussion
National Weather Service Huntsville AL
903 PM CDT Wed May 24 2017

.NEAR TERM...(Tonight)
Issued at 903 PM CDT Wed May 24 2017

A rather highly amplified upper level pattern continued over and near
the greater North American domain. A feature influencing local 
weather was an upper low approaching the Tennessee Valley from the 
Mid South. Both GOES 13 and 16 imagery nicely show this mid/upper 
low. 25/0000Z 500mb heights around this low were near 551 decameters,
which is pretty low for this time of year. This has resulted in very
steep mid level lapse rates, which has lead to short lived, isolated
thunderstorms within shower bands near and southeast of this upper 

Local and regional radar views indicated scattered to at times
numerous showers moving in a west to east manner across the forecast
area. The back edge of these showers were over Mississippi River
between NE Arkansas and around Greenville MS. A few lightning 
strikes (most of them intercloud) have been accompanying this 
activity, seen from local LMA and MRMS. 

For the rest of the overnight, the above mentioned upper low will
continue moving a bit south of east across the forecast area. Output
from the HRRR indicated 1000/500 thickness values below 550
decameters as this system swings by, especially between midnight and
3 to 4 AM. Shower activity with this system per the HRRR and RAP
should end across our eastern areas before daybreak Thu, with the 
new NAM a bit slower with the drying trend. 

Update wise, stayed close to previous forecast trends/timing. Did 
lower rain chances over our eastern areas late tonight more in line 
with the drier high resolution guidance. Cooler surface temperatures
(lows falling into the lower 50s and the already cold air aloft 
should lessen lightning development after 10 PM, as lapse rates aloft
moderate somewhat.

.SHORT TERM...(Thursday through Friday night)
Issued at 304 PM CDT Wed May 24 2017

Thursday will be another relatively cool day for late May, with
consensus highs only reaching the low-mid 70s (normals are 83). There
is still an outside chance of a shower in northeast Alabama, since a
last-gasp shortwave will round the southwest flank of the departing
low. However, with precipitable water values less than an inch over
that region, the forecast will be left dry. One more below-normal
night is expected Thursday night with highs falling to the mid 50s.

The cool spell will come to an abrupt end as abundant sunshine,
strong SW flow and subtle mid-level ridging all kick in Friday. 
Highs should easily reach the mid-upper 80s, with a nudge just above
blended guidance. A cold front will begin a slow southward push late
Friday into Friday night, but it should not get close enough to the 
Tennessee Valley to warrant including any PoPs in the forecast 
through Friday night.

.LONG TERM...(Saturday through Tuesday) 
Issued at 304 PM CDT Wed May 24 2017

The latest med range model output continues to show deep SW flow 
prevailing from the Mexican Plateau to the Southeast and Ohio River 
Valley on Saturday. Meanwhile, a surface warm front will be lifting 
north on Saturday afternoon in response to an approaching mid-level 
shortwave trough moving within the subtropical jet. This will occur 
in conjunction with a polar jet trough rotating around a parent 
upper low moving southeast over Corn belt and Great Lakes region. 
That particular trough will become important for later in the 
evening/overnight Saturday night. 

For Saturday, scattered storms may occur as the warm front moves 
north during the late morning into early afternoon. Plan view of 
instability parameters are showing a moderate to high amount of 
instability during the afternoon, but forecast soundings indicate 
most of this is elevated due to an EML present likely from the fetch 
off the Mexican Plateau.  Because of this limiting factor, will keep 
scattered storms in the forecast, but this activity could have even 
less coverage than forecast.  As the warm front moves north, the 
aforementioned shortwave trough will be pivoting southeast towards 
the MS River Valley with a trailing cold front. The ensuing 
confluence of the flow aloft will act to increase geostrophic flow 
across the MS/TN/OH river valleys for Saturday night. Upstream (NW 
of here) deep convective initiation is expected to increase in 
coverage and likely form into a MCS rapidly moving southeast towards 
the TN Valley on Saturday night. Deep layer shear is around 40-50 
kts especially after 7PM. Most unstable CAPE (2000-3000 J/kg) is 
mostly in the elevated portions of the profile (within the hail 
growth zone) which would heighten the potential for large hail along 
with heavy rainfall once the profile becomes saturated.  Though some 
uncertainty exists on the trajectory of the MCS, it should spread a 
cold pool across the TN Valley which may limit convective activity 
during the day on Sunday. However, additional showers and storms 
should develop along the trailing surface cold front that should 
move over the region sometime late Sunday night into early Monday 

The latest med range models runs do show a slower trend with the 
cold front passage which is plausible with the bulk of the upper 
level forcing with the trough associated with the parent upper low 
removed further to the north. So, the precipitation may not 
completely clear before sunrise on Monday morning and may affect 
Memorial Day outdoor activities. The temperatures should be cooler 
behind the front for both Monday and Tuesday. Dry conditions should 
prevail on Tuesday as well. 


.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 630 PM CDT Wed May 24 2017

Mainly VFR conditions should continue through TAF. That being said, 
an area of showers and a few embedded thunderstorms will continue 
moving in a west to east manner across the area this evening. CIG 
and/or VIS values may briefly be reduced to MVFR, but chances of it 
are too low to include in the TAF. Stayed with a timing of the shower
activity ending around 07Z at KMSL and 08Z at KHSV. VFR altitude 
lower clouds should persist for the rest of the night and during Thu 
AM. As the coldest air aloft moves east of the region, expect a 
broken cloud deck to scatter in the early afternoon.





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