Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 292014

Area Forecast Discussion
National Weather Service Huntsville AL
314 PM CDT Mon May 29 2023


(Remainder of today and tonight)
Issued at 314 PM CDT Mon May 29 2023

Latest visible satellite data and regional ASOS/AWOS observations
indicate that a broad plume of lower-level stratus clouds (to the
west of a deep-layer cyclone currently located along the eastern
border of NC-SC) has built back into the TN Valley from the
northeast this afternoon. We have reduced the current period max
temp forecast by a few degrees to account for this, but otherwise,
there have been no significant changes to forecast reasoning for
the remainder of the day since the morning update.

Overnight, the weakening upper low to our east will drift slowly
east-northeastward in the vicinity of the Outer Banks of NC. As
this occurs, a broad region of weak steering currents aloft will
set up across the TN Valley between the low to our east, a mid-
level ridge centered across southern Ontario and a shortwave
trough spreading southeastward off the TX Gulf Coast. At the
surface, northerly winds will quickly become light/variable this
evening as our CWFA will positioned along the southwestern edge of
a high pressure cell building southeastward into New England.
Presuming that the stratus layer erodes from W-to-E during the
late afternoon/early evening period, patchy fog may develop in
portions of the region as temps fall into the u50s-l60s.


(Tuesday through Wednesday night)
Issued at 314 PM CDT Mon May 29 2023

During the short term period, model consensus suggests that the
local area will remain beneath a region of weak steering currents
between a decaying upper low located off the Mid-Atlantic coast, a
strengthening 500-mb ridge centered over the eastern Great Lakes,
and a broad region of cyclonic flow aloft encompassing much of
the Gulf of Mexico. In the low-levels, dewpoints will quickly
recover to levels more typical of the season (l-m 60s) by tomorrow
as northerly flow/dry advection subside in the wake of the
departing cyclone to our east, and this will support the
development of a few afternoon showers and thunderstorms. Our best
guess is that this will occur as a ridge (attached to the high to
our northeast) builds southward in the lee of the Appalachians,
inducing a subtle low-level convergence axis that will drift
westward into the region late tomorrow afternoon. Shear will be
very weak, and with afternoon temps in the l-m 80s only supporting
CAPE in the 500-1000 J/kg range, lightning and locally heavy
rainfall will be the main threats with the storms. High-res data
from the 12Z CAMs indicates that at least isolated convection can
be expected through the evening hours tomorrow, with mild temps in
the l-m 60s and patchy fog expected Wednesday morning. Little
change in the synoptic pattern is anticipated Wednesday/Wednesday
night, although a gradual increase in both deep-layer moisture and
instability will likely lead to a greater coverage of showers and
storms that should once again continue into the evening hours.


(Thursday through Monday)
Issued at 314 PM CDT Mon May 29 2023

Within a broad weakness in the mid-level height field left behind
by the departing east coast upper low, it appears as if a new
closed cyclone may become established off the southeastern
Atlantic Coast early in the extended period (Thursday-Friday).
Should this occur, northerly flow aloft will strengthen across the
TN Valley, with the southward advection of a drier mid-level
airmass into the region leading to a gradually lower coverage of
afternoon convection both days. This will unfortunately translate
to an increase in temperatures, with highs expected to rise into
the 85-90F range for most of the CWFA on Friday. Although there is
considerable uncertainty regarding the track and speed of the
southeastern Atlantic Coast upper low beginning this weekend, it
appears if the region will remain beneath moderately strong
northeast flow aloft along the south side of a mid-level high
retrograding across the Great Lakes and into the northern Plains.
This will in all likelihood maintain dry profiles aloft and a
limited coverage of afternoon showers/storms, although we will
leave a very low (15%) POP in the grids each day from Saturday-
Monday. Highs will continue to rise, reaching the u80s-l90s on
Saturday, and similar temps may continue for several days in spite
of blended guidance suggesting otherwise. Lows may fall a degree
or so based on trends in dewpoints, but should generally remain in
the l-m 60s.


(18Z TAFS)
Issued at 1157 AM CDT Mon May 29 2023

Current satellite data indicates that the western edge of wrap-
around stratus clouds (associated with an upper low to our east)
will continue to erode across northeast AL this aftn. Modest
warming of the boundary layer will support the development of sct
Cu/Ac at the terminals, which should dissipate shortly before
sunset, leaving only sct Ci for much of the evening. Stratocu will
begin to return to the region around or shortly after Midnight as
winds in lower-levels begin the process of veering from NE to
ESE, with development of cigs arnd 5 kft possible by the end of
the TAF period. Some patchy BR/FG may also develop invof the
terminals from late this evening-sunrise tomorrow, but with
uncertainty regarding the evolution of early morning clouds, we
will not include vsby reductions in the forecast attm.





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