Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 170900

Area Forecast Discussion
National Weather Service Birmingham AL
400 AM CDT Tue Jul 17 2018

Today and Tonight.

An overall northwesterly flow pattern continues for today and
tonight as an upper level ridge remains in place across the Western
and Central CONUS with a trough sliding across the East Coast. A
week frontal boundary is expected to push into the area, but models
have been trending drier with the overall PoPs in latest runs. I
would expect at least some convective initiation, similar to what
the a few of the high-res CAMS develop along the convergent
boundary, but where that initiation takes place and how long it
lasts will be difficult to nail down. Therefore, I have kept 40-50%
chance of rain in the forecast for this afternoon with the morning
remaining mostly dry.  PWATS exceed 2" by this evening when the best
rain chances would exist, given model timing of the frontal
boundary. This could lead to localized heavy rainfall with any
storms that are able to develop.

Aside from the rain chances, another humid day is in store across
Central AL with heat indices nearing 100 with some locations in
the western portions of our area just edging over 100 due to
slightly higher dewpoints. Cloud coverage should be high enough to
keep us out of any heat advisory conditions, however, so will not
add any heat impacts to the HWO today.


Wednesday through Monday.

Northwest flow will remain in place aloft on Wednesday between a
mid-level trough extending from New England southward to the
northern Gulf, and a mid-level ridge extending from the Southern
Plains westward to the Pacific. A shortwave ridge will also be in
place over the Midwest, ahead of a shortwave trough over the
Northern Plains. Northerly low-level flow associated with high
pressure over the Great Lakes will allow the cool front to sink
southward towards the I-85 corridor afternoon. The ECMWF has
actually trended towards the other models in showing some drier
air mixing down across the northern counties during the afternoon.
This will set up a sharp moisture/PoP gradient with scattered to
numerous showers/storms across the south and rain-free conditions
across the north. Heavy rainfall/localized flooding will remain
possible in the high PWAT airmass along/south of I-85/Highway 80.
Dew points look to mix down into the 60s over at least some of the
northern counties, but temperatures will still hit 90 in most
spots. Isolated to scattered showers/storms will remain possible
Wednesday night in the far southern counties near the front. Any
MCS that develops over the Plains/Ozarks would likely remain well
to the west over the ArkLaMiss.

On Thursday, the Northern Plains shortwave/upper low will move
into MN while a lead shortwave will extend southward towards
northwest Alabama. A wave of low pressure more noticeable at 850mb
than at the surface will develop in the area of convection along
the stalled front along the Gulf Coast. The ECMWF has trended
slower with the development of this feature more in line with the
other models, and therefore the model consensus now indicates the
front will be a bit slower to lift back north as a warm front.
This will keep the drier air mass in place longer across the area.
PoPs were reduced and may be reduced further in future updates in
north-central/interior portions of the area. Best rain chances
will be in the far southeast closest to the stalled front.

The GFS and ECMWF remain in some disagreement for Friday. The
ECMWF is quicker to lift out the wave of low pressure up the East
Coast and therefore has more moisture return compared to the GFS.
Will go with PoPs in the scattered category areawide. Any storms
that do develop Friday afternoon could be strong with 0-6 km bulk
shear values of 20-25 kts, dry air aloft, and increasing mid-
level lapse rates.

Troughing will continue to amplify over the eastern CONUS Friday
night into Saturday, as an unseasonably strong vertically stacked
low moves into the Great Lakes with a trailing cold front. An
area of unseasonably strong northwesterly mid and upper-level flow
will be located along its southern flank. Friday night, there will
be the potential for one more more MCSs to form along the front.
The best upper-level forcing will remain north of the area, but
Corfidi vectors would support some activity making it into our
northern counties. If an MCS or organized cluster is able to hold
together when it arrives Friday night, 0-6 km bulk shear values of
30-35 kts and strengthening mid-level lapse rates would be
supportive of the potential for it to be strong to severe.
However, confidence in the strength/track of any MCSs this far out
is too low to mention in the HWO at this time, especially given
the nocturnal timing.

The cold front will move into the area on Saturday. Unusually
strong (for late July) 0-6 km bulk shear values of 30-40 kts and
decent mid-level lapse rates would be supportive of a threat of
strong to severe storms assuming sufficient instability. Main wild
cards are outflow from the possible MCS Friday night and what
impact that could have on destabilization. Some drier air may move
in aloft which could limit storm coverage. Given the uncertainty
this far out, will not add a severe mention to the HWO to this
time, but one could be added in future updates if trends continue.
The potential main threat would be damaging winds.

A drier air mass looks to move into much of the area
Sunday/Monday as the trough remains over the eastern CONUS,
resulting in reduced rain chances, dependent on where the front
stalls and residual boundary layer moisture.



06Z TAF Discussion.

The rain showers are tapering down for the night with only a few
showers near EET/ASN/ANB possible for the next couple of hours. We
remain in a wet pattern with persistence remaining a factor. Low
stratus are expected for many and some areas of fog are possible
especially in areas that had rain during the day. More afternoon
TSRA are possible once again for Tuesday.




Above normal coverage of mainly afternoon and early evening
showers and thunderstorms is expected again today as a weak front
moves towards the area. Highest rain chances will become confined
to the southern counties Wednesday and Thursday as the front
stalls. Localized fog will be possible each morning where
rainfall occurs during the previous afternoon and evening. There
are no fire weather concerns.


Gadsden     89  72  92  67  92 /  50  40  10   0  20
Anniston    91  73  92  69  92 /  40  40  10   0  20
Birmingham  91  75  92  71  92 /  40  40  10   0  20
Tuscaloosa  91  74  94  72  94 /  50  40  10  10  20
Calera      90  73  91  70  92 /  40  40  10  10  20
Auburn      89  75  90  71  90 /  40  30  50  20  30
Montgomery  91  75  92  73  93 /  40  30  60  30  30
Troy        90  75  90  72  92 /  40  40  70  40  50




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