Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 251057

Area Forecast Discussion
National Weather Service Birmingham AL
557 AM CDT Mon Sep 25 2017

For 12Z Aviation.


Today and Tonight.

An area of light rain has developed over southwest Alabama due to
weak mid-level isentropic lift centered on the 315K surface. This is
related to southeasterly mid-level flow around the weak mid-level
low centered near the Mississippi Gulf Coast and possibly a remnant
MCV from yesterday`s MCS over the Gulf. Cloud bases are probably
around 12-15k ft in this area, a bit lower than the 17k ft cloud
base observed on the special 6Z sounding in support of the Hurricane
Maria forecast. So, most spots in the far southwest counties will
probably not measure but will get some trace rainfall amounts. Added
in some PoPs for the rest of the night in this area, adding on to
today`s daytime PoPs, with mid-level moisture continuing to increase
and additional activity lifting northward from the coast. Elsewhere,
mid and high level clouds are streaming northward due to southerly
flow aloft and blow off from Gulf convection.

The weak upper low that has been meandering in a circuitous route
around the eastern CONUS for the past few days (this feature can be
tracked back to the shortwave moving through the Ohio Valley almost
a week ago that produced fairly widespread rainfall last Tuesday)
has ceased its westward progression and is beginning to drift back
to the east. It is being squeezed between two ridges and by the
approaching west coast trough and Maria to its east, causing it to
open up into more of a trough axis. The energy will be splitting
with the southern portion re-forming an upper low center further
south over the Gulf. The mid-level low will remain fairly stationary
near the Mississippi Gulf Coast. These features will continue to
contribute very little in the way of forcing with just a couple weak
vort maxes moving over the area as they rotate around the upper low,
but aid moistening. Southerly to southeasterly flow will allow Gulf
moisture to lift northward aloft, but low-level ridging centered
over the Northeast will maintain low-level easterly flow across
Central Alabama, preventing richer Gulf moisture from being lifted

Highest rain chances today look to be closer to the Gulf Coast.
However, the HRRR and NSSL WRF now show isolated development by late
morning across the western two thirds of Central Alabama. This makes
sense with increasing mid-level moisture, better layer mean RH
values, and PWATs around 1.5 to 1.6 inches, increasing to 1.8 inches
in the southwestern counties. PWATs increased from 1.33 to 1.46
inches between the 0z and 6z soundings. So the area of isolated PoPs
was expanded across the area, and added some scattered PoPs in the
southwest. Additional updates may be needed through the day. The mid-
level cold core weakens substantially today, with limited instability/
skinny CAPE profiles across the area. Therefore, will limit the
mention of thunder to far southern areas with just showers expected
elsewhere. Not expecting any storms to be particularly strong today
with profiles aloft becoming warmer and more moist. Additional
cloud cover should keep highs from reaching 90 in most areas.
Drier air works in from the east late this afternoon and tonight,
and will continue with a dry forecast after sunset.


Tuesday through Sunday.

Forecast for Tuesday into Wednesday remains on track. The main
upper low associated with the Rockies system moves northeast
toward Manitoba Tuesday and toward Quebec by Wednesday. At the
same time, a piece of energy breaks off from the upper trough
forming a closed low on Wednesday over the Desert Southwest.
Ridging fills in over the Gulf behind a weakening Maria (as she
moves northward along the Atlantic Seaboard over the Atlantic) and
starts to wrap its way around into Texas and the Lower MS River
Valley. We should remain dry for Tuesday into Wednesday across
Central Alabama being on the dry northerly flow side of Maria
along with ridging in place across much of the Deep South except
for Florida where some weak troughing extends from Maria to the
SW into the NE Gulf.

At the surface, a cold front makes progress across the central
part of the US expected to move through late Wednesday into early
Thursday. Guidance still looks like not enough moisture for any
precipitation since we are not getting return flow and available
moisture in place ahead of time. The second reinforcing surface
front moves in behind it on Friday into Saturday. Extended guidance
is coming into agreement for the weekend. Behind the front, a
slightly weaker 1028-30 MB ridge center moves down out of Central
Canada across the Plains and into the Upper Mid West by early
Saturday with some N/NE flow into Alabama. From there, the ridge
does look to elongate into Quebec and flatten across the Deep
South with NE/E flow into the state by late Sunday as Maria should
be moving away from the US mainland by then.

Overall, we are still looking at a fairly nice weekend with drier
and cooler conditions but with a slightly weaker secondary front
with a flatter ridge possible. The slightly milder (by a couple
of degrees or so) than originally thought scenario that I wrote
about yesterday may end up winning out. We will definitely see
70/80s for highs and some 50s to mid 60s for lows which is a
definite improvement and more climatologically expected for late



12Z TAF Discussion.

VFR conditions are expected through the period. Mid and high level
clouds are streaming over the area due to an upper low near the
Mississippi Gulf Coast. This moisture will result in isolated to
scattered showers and thunderstorms during the late morning and
afternoon hours. Coverage is currently expected to remain too
isolated to mention in any TAF except MGM where VCSH was
introduced. Amendments will be made as necessary. Winds remain
easterly at 5-8 kts.




Look for isolated to scattered shower and thunderstorms today
with the best chances in the southwest counties of Central Alabama
this afternoon with daytime heating. Relative humidity values
only drop into the upper 40s and 50s. There are no fire weather
concerns at this time. A change in the pattern is expected by
early Thursday with the arrival of a front along with cooler and
drier weather expected as we head into the weekend.


Gadsden     86  63  88  64  89 /  10   0   0   0   0
Anniston    87  65  87  65  88 /  20   0   0   0   0
Birmingham  87  68  89  68  90 /  20   0   0   0   0
Tuscaloosa  88  67  91  68  91 /  20  10   0   0   0
Calera      87  67  89  67  90 /  20   0   0   0   0
Auburn      86  66  88  68  89 /  10   0   0   0   0
Montgomery  89  69  90  69  92 /  20  10   0   0   0
Troy        87  66  89  67  90 /  20  10   0   0   0




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