Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 190128

Area Forecast Discussion
National Weather Service Birmingham AL
828 PM CDT Wed Jul 18 2018

For 00Z Aviation.


Through Tonight.

Isolated to scattered thunderstorms have developed mainly along
and south of a low level boundary. Activity across Central Alabama
has little or no upper support, and should dissipate early this
evening. Slightly more organized activity just east of the
Mississippi River may bring some storms to far west Alabama this
evening, and kept low rain chances for this area and areas along
and south of I-85. Expect rain free conditions by midnight. Very
muggy conditions currently with surface dewpoints in the mid 70s.
High levels of low level moisture will keep overnight lows on the
warm side.


Thursday through Tuesday.

A split upper-level flow pattern will temporarily be in place
across the eastern CONUS Thursday with a broad trough/area of
cyclonic flow aloft over the Southeast and a ridge over the
eastern Great Lakes. This will be downstream of a strong
northwesterly upper-level jet streak nosing into the Central
Plains, with a vigorous shortwave located to its north over the
Midwest and a strong ridge to its south centered near the Four
Corners. At the surface, high pressure will be building down the
lee of the Appalachians while a weak wave of low pressure develops
near the Gulf Coast. This will will result in easterly winds
building into East Alabama with an increase in low-level moisture
and convergent flow. Meanwhile the stalled frontal boundary will
become more diffuse. Models have trended quicker with moisture
return, and combined with a weak vort max aloft coverage of
afternoon showers/storms looks to be greater than previously
expected. Therefore PoPs were raised especially in the
southern/eastern counties. Activity should mainly diminish after
sunset, but there is a slight chance that remnants of convection
developing upstream could reach the northwest counties after

By Friday the southern stream upper trough will push off to the
east as the ridge over the Southern Plains begins to build
westward. The northern stream shortwave with embedded upper low
will move into Wisconsin. 500mb heights will rise over Central
Alabama with associated synoptic-scale subsidence, while 700mb
temperatures warming to +10 to +11C will result in increased
capping. This may suppress diurnal convection except in the
southeast counties. However, will keep in at least a slight chance
everywhere given available PWATs and the potential for lingering
boundaries. Strengthening northwest flow aloft results in 0-6 km
bulk shear values around 35 kts with increasing mid-level lapse
rates. There will be a conditional risk of an isolated strong to
severe storm Friday afternoon if convective initiation can occur.
Low-level flow will become westerly over West Alabama, with
trajectories originating from a very hot air mass over the
Southern Plains. This will result in heat index values around 105
degrees, and this threat will be added to the HWO.

One or more MCSs/strong to severe clusters of storms are expected
to develop off to our north Friday afternoon/evening due to
forcing associated with the upper low and a cold front at the
surface. A "ring of fire" type pattern will be in place around the
southern Plains ridge and associated strong capping/EML. The best
height falls/forcing will remain northeast of our area. There is
the possibility that some of this activity could at least clip our
northeast counties after midnight, but models disagree on this.
There is the possibility that this activity could be strong to
severe given the deep layer shear/lapse rates, but given great
uncertainty in the track/strength of this activity and the
overnight time frame, confidence is too low to mention in the HWO
for Friday night.

Troughing continues to amplify over the eastern CONUS on Saturday
as a cold front approaches from the north. NAM/GFS wind fields
appear contaminated by convective feedback, but even the ECMWF
shows northwesterly 0-6km bulk shear values greater than 40 kts
over Central Alabama, which are very unusual for late July. Given
the presence of a moist and unstable air mass and favorable lapse
rates, this will result in the potential for strong to severe
storms capable of producing damaging winds and hail. However, the
big caveat is what impact convection late Friday night continuing
into Saturday morning will have on available instability. Both the
GFS and ECMWF show a decent amount of QPF between 12 and 18Z
Saturday morning. This could contaminate what would otherwise be a
favorable northwest flow severe event. Therefore, while a threat
of severe storms will be added to the HWO, will keep it as a low
confidence threat for now. Will also note that there will be some
SRH present, and given the deep layer shear values the
possibility of a tornado cannot be ruled out. But again given the
low confidence associated with the potential for disruptive
morning convection, will not mention this particular threat in the
HWO at this time. Also, if a cold pool does not form in the
morning, heat index readings could reach 105 as well.

The eastern CONUS trough will remain the main player in the rest
of the extended forecast period. Its placement will have a big
impact on temperatures and dew points. Moisture wrapping around
the trough may be enough to spark some showers/storms at times
Sunday through Tuesday.



00Z TAF Discussion.

Showers and storms are beginning to wane this evening. A few
locations start with VCSH/VCTS, but after 02z, most areas remain
rainfree the remainder of the night. A stationary front remains
draped over the region with higher moisture content south. It
appears that some low level isentropic lift will be in play south
and as the sun rises, IFR ceilings and fog will affect MGM/TOI.
These clouds lasted a long time today and am not expecting that at
this time. Farther north, mentioned patchy MVFR fog.

On Thursday, showers and storms will be most numerous south and
have added PROB30 south in the afternoon.




Scattered to numerous showers and storms are expected today in the
southern counties with activity remaining isolated further north.
Scattered showers and storms are expected again on Thursday with the
most coverage south and east. Localized fog will be possible each
morning where rainfall occurs during the previous afternoon and
evening. There are no fire weather concerns.


Gadsden     70  91  70  91  72 /  20  30  20  20  50
Anniston    70  90  71  91  72 /  20  40  20  20  40
Birmingham  75  93  73  93  75 /  30  30  20  20  40
Tuscaloosa  73  94  73  95  76 /  50  30  20  20  30
Calera      73  90  71  92  74 /  40  40  20  20  30
Auburn      73  88  71  88  72 /  30  50  40  40  30
Montgomery  73  92  73  94  75 /  50  50  30  30  20
Troy        74  90  73  92  74 /  40  60  40  40  20




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