Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 152339

Area Forecast Discussion
National Weather Service Birmingham AL
539 PM CST Fri Dec 15 2017

For 00Z Aviation.


Through Tonight.

Northerly winds have been breezy this afternoon behind the
exiting cold front, with an extensive cirrus shield. This has kept
temperatures in the upper 30s to mid 40s. For tonight, surface
high pressure will move across the area, and winds will diminish.
Upper level jet stream remains active across the southeast with a
moist fetch spreading northeastward from the Pacific and across
Mexico. This will maintain the cirrus cloud cover across the area
tonight, and have adjusted temperature trends to account for
slower cooling after sunset. Clouds should thin toward sunrise to
allow for quicker cooling with temperatures falling into the upper
20s north and lower 30s south.

The only change made to the extended forecast was to increase
pops Tuesday and Wednesday to account for the wetter trend in
model solutions. No other changes were needed with this forecast
package and details in the discussion below remain valid.


Saturday through Friday.

Saturday/Saturday night:
Upper-level ridging will temporarily build over the area on
Saturday, downstream of the cutoff upper low currently over
northwest Mexico that will be ejecting northeastward into Texas as
a shortwave trough in the southern stream. A dry air mass will
remain in place during the day on Saturday with only a steady
stream of high clouds expected. Low-level flow above the surface
will switch to southwesterly as low-level ridging moves eastward.
But at the surface, high pressure initially centered over the area
will only slowly drift eastward towards the Atlantic coast,
keeping surface winds out of the southeast, and the better warm
air advection will hold off until Saturday night. This should keep
high temperatures mainly in the 50s.

Deep layer southwesterly flow will increase substantially
Saturday night as the shortwave moves into the ArkLaTex region
with strong warm air and moisture advection as well as isentropic
lift just above the surface. However a very dry air mass will
remain at the surface. A large complex of showers will develop
over Texas and move eastward in association with a 40-50 kt LLJ,
as a weak surface low that develops along the stalled front in the
western Gulf also lifts northward. Most models agree that this
activity will remain in Mississippi prior to 12Z, but with
increasing moisture in the west and the potential for either
isolated to scattered showers ahead of this activity, or a
possible faster timing, have kept in some lower rain chances in
the west after 3AM. This also agrees with SREF probabilities and
other ensembles. A strong warm nose aloft will keep precipitation
all liquid. Low temperatures will probably be reached between
midnight and 3 AM, before rising slightly towards sunrise. Mainly
upper 30s to low 40s are expected, with some low to mid 30s in the
cooler northeastern areas.

The negatively tilted shortwave will lift quickly off to the
northeast, reaching the Ohio Valley by midday while grazing
northwest Alabama during the morning hours. The remnants of
Saturday night`s activity should move into western and northern
portions of the area Sunday morning, maintained by the low level
jet. It may weaken with eastward extent by midday as the LLJ
weakens and the upper-level forcing lifts to the northeast. As
this rain falls into the remnant dry air mass at the surface,
temperatures may remain in the 40s across the northwest due to
evaporative cooling. The main focus for shower activity will shift
to the southwest counties by Sunday afternoon, as additional
activity developing near the coast lifts northeastward in the deep
layer southwesterly flow amid increasing deep layer moisture and
PWATs. A surface warm front will also attempt to lift northward,
but its northward progression will be slowed initially due to
evaporative cooling as precipitation falls into the dry air mass
north of the front. This will all result in a tricky temperatures
forecast, as highs may struggle to reach 50 across the far north,
while reaching the upper 60s in the far southeast. Models indicate
very little in the way of MUCAPE and will keep a mention of
thunder out of the forecast for this period.

Sunday night through Monday night:
The ECMWF has generally trended towards the GFS/Canadian and EPS
ensemble mean with keeping more of a phased trough over the Desert
Southwest and keeping a moist southwesterly flow across the area
for the first half of next week. Yesterday`s 12Z ECMWF trended
wetter but then tonight`s 00Z run trended drier at least for the
Monday/Monday night period, with uncertainty over the position of
the front and dry air to its north. Overall though, confidence is
increasing that this period will be wet and unsettled, and will
stick closer to the GFS/Canadian during this period which also has
the support of the EPS ensemble mean. Continued deep layer
southwesterly flow between the trough over the Desert Southwest
and a strong ridge near the Bahamas will allow the warm front to
move northward Sunday night into Monday, and temperatures should
rise through the night Sunday night in most locations. Deep layer
moisture, isentropic lift, and broad upper-level lift from the
right entrance region of an upper-level jet streak will promote
high rain chances. Some weak instability will develop which will
allow for a couple thunderstorms as well. Depending on
precipitation trends, if some pockets of heating develop highs
could be near 70 in the southern counties Monday afternoon.
Couldn`t rule out an isolated strong strong to severe storm with
gusty winds/small hail during this time period in the southern
counties given some instability and deep layer shear. But
warm/saturated profiles aloft, limited low-level shear, and lack
of a focusing mechanism are expected to prevent any organized
threat of severe storms.

PWATs in the 1.5 to 1.8 inch range for a somewhat prolonged period
of time, near the climatological maximum for December, and
unidrectional flow does raise some concern for cell training and
locally heavy rainfall. Current WPC QPF indicates area-averaged
amounts of 1 to 2.6 inches through Tuesday. Flash flood guidance
is high due to recent dry conditions with drought currently in
place across the western counties. Streamflows are running below
normal in the west and near normal elsewhere. Most areas could
handle 1 or 2 rounds of heavy rain, but will have to monitor for
flooding if multiple rounds of heavy rain occur. Expect some
localized flooding in poor drainage areas, but confidence in any
widespread flooding is too low to mention in the HWO at this time.
Will continue to monitor QPF trends closely, however.

Tuesday through Wednesday:
Models disagree on whether the trough will push eastward across
the area in one or 2 pieces. Will continue to go with the GFS idea
of a cold frontal passage on Tuesday with drier air for
Wednesday, but the latest ECMWF and its ensemble has trended
wetter for Wednesday so will have to see it that trend continues.
Similar to Monday a conditional potential for an isolated strong
to severe storm exists along the front if some instability
develops, but weak/veered low-level flow does not appear
supportive of any organized potential.

Models seem to be coming into better agreement on another trough
moving into the western CONUS during the second half of the week,
with a strong cold front moving through on Friday. Still too far
out to determine any details regarding whether or not there will
be any threat of severe weather. Also, at this time,
precipitation is expected to move out before any cold air arrives,
but this is still a week away.



00Z TAF Discussion

VFR conditions remain over Central Alabama through the period. A
surface high pressure area moves west to east through Saturday
night. This will allow initial northwest winds to become light
though Saturday morning, then swing around to a southerly
component by Saturday afternoon. Even with this change in
direction, winds will remain on the light side. Southwest flow
aloft will keep considerable high cloudiness overhead. The lower
levels remain very dry and no clouds are expected below 15k ft.
Moisture increases as winds around 2k ft increase Saturday night
into early Sunday.




No rain is expected until late Saturday night or Sunday. Relative
humidity values will remain well above critical thresholds. A wet
pattern is expected beginning Sunday through the first half of next


Gadsden     27  54  34  51  49 /   0   0  10  70  70
Anniston    28  55  36  55  52 /   0   0  10  60  70
Birmingham  30  54  39  53  52 /   0   0  10  80  70
Tuscaloosa  30  56  40  54  54 /   0   0  20  80  70
Calera      28  54  39  56  55 /   0   0  10  80  80
Auburn      31  56  39  61  55 /   0   0   0  40  80
Montgomery  30  56  40  61  58 /   0   0  10  60  80
Troy        30  57  41  66  59 /   0   0   0  40  70




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