Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Birmingham Alabama
715 am CDT Mon Mar 19 2018
for dense fog expansion.
today and tonight.
Confidence is increasing in the severe weather potential for this
afternoon and evening. The threat for tornadoes has also increased,
and a strong tornado or two will be possible across northern
portions of central Alabama as well. It now appears that there may
be two waves of severe weather, with a first wave during the
early/mid afternoon hours which is more isolated/conditional,
followed by the main wave during the late afternoon/evening hours.
The severe weather starting time was bumped earlier to 2 PM to
account for the first wave, while the second wave has trended a bit
slower resulting in the ending time being pushed back to 2 am,
mainly for southern/eastern areas. More on these waves below.
A lead shortwave is currently pushing into western portions of the
state, indicated by pronounced drying in mid-level water vapor
imagery. This is producing some light showers north of the warm
front, with an mesoscale convective system now well southeast of the area over the Florida
Panhandle and southern Georgia. The radar is clear over southern MS and
la, suggesting the warm front should not have much trouble lifting
northward. Scattered clusters of warm advection thunderstorms are
located over northern MS and western Tennessee. It's not out of the
question that these could clip far northwest portions of central
Alabama during the early morning hours with the possibility of hail.
Further west, a broken pre-frontal line of storms is located over
far eastern OK and into far NE Texas ahead of the main upper low of
interest. These are expected to lift well northeast of central Alabama.
Will note that they extend a bit further south than expected, so
that is something to monitor, but the southern end will probably
weaken as it encounters a more stable air mass over Arkansas and northern
la. The surface low is located over northern OK, with an occluded
front extending southward to a triple point near where OK/AR/TX come
together. A cold front/dry line extends southward from the triple
point. A warm front extends eastward all the way into far southern
Patchy fog will be possible this morning along and north of the warm
front with a low stratus deck over the area as well. The warm front
should lift northward this morning with broken sunshine developing.
This will result in heating, and combined with moist low-levels and
steep mid-level lapse rates associated with an eml, cape values of
2000-2500 j/kg will develop, with 60 kts of 0-6 km deep layer shear.
This high cape/high shear parameter space is supportive of a
significant severe threat. Models indicate a weak wave at 700mb
lifting across the area over the open warm sector during the
afternoon hours, ahead of the main upper-level forcing associated
with the upper low and the cold front/dry line at the surface.
Almost all models show some light quantitative precipitation forecast developing with this feature,
but differ regarding whether there will be any deep convection or
just scattered showers. The 3km NAM and a couple runs of the hrrr
suggest isolated convection/supercells are possible. This will be at
peak heating and the cap is not particularly strong, so it at least
seems plausible. All modes of severe weather would be possible with
any isolated discrete supercells that do form. Also of note is a
well defined fine line on radar that the storms in northeast Texas are
forming on, which may be indicative of an outflow boundary/bore,
which could cause early initiation as it moves eastward. This first
wave complicates the timing graphic, resulting in a much broader
threat timeframe, with the first wave starting as early as 2 PM.
It's also unclear what effect this first wave will have on the
second wave, but current thinking is that it won't have much of an
The second main/most widespread wave of storms will develop over
northern MS/northwest Alabama as height falls and the mid-level speed Max
associated with the upper low and low amplitude trough interact with
the dry line at the surface. Shear profiles favor a fast-moving
broken line of supercells that will race eastward across the area
during the late afternoon and evening. Model consensus excluding the
GFS now indicates that winds ahead of this line will remain backed
in a southerly direction, which is also indicated in the href mean
winds. This results in greater 0-1km srh and more favorable critical
angles. The line of supercells looks to cross the area during the
favorable early evening transition period of increased low-level
shear. Therefore, it appears that the threat for tornadoes including
the potential for a strong tornado or two across the northern half
of central Alabama has increased. This will ultimately depend on
storm scale details and what effects the first wave has. The threat
of large hail, including significant hail >2" diameter continues
given the fat cape profiles, and many sars significant hail analogs.
Isolated hail up to baseball size may be possible. The threat will
end by midnight for many, except for southeast and far south-central
portions of central Alabama where it will continue through around
2am. The enhanced risk area in our local impact graphics remains on
track, and lines up well with the >40% neighborhood probability of
updraft helicity > 75 m2/s2 in the href cam ensemble. Not everyone
will see severe storms today given the scattered nature of the
storms, but those that do will see some intense storms. Everyone
should remain weather aware.
Tuesday through Sunday.
As the main wave of showers and storms exit the area, the main
upper low will swing into the area on Tuesday and slowly work
through the area. Scattered light showers will be possible
throughout the day Tuesday and into Tuesday night along with
cooler temperatures. Overall the consensus of the models clear the
rain out around midnight, before the temperatures can drop into
Drier air filters into the region on Wednesday and will hang out
for several days. Temperatures will be on the cooler side with
highs in the 50s and lows in the 30s and 40s. We will need to
monitor Wednesday and Thursday morning for possible freezes, but
Thursday appears most probable for at least a frost across the
northern 2/3rds of the area. Will continue to mention the cold in
the severe weather potential statement for Thursday morning. Temperatures will continue to modify
through Friday with highs in the 60s on Thursday and 60s/70s on
Rain will begin to filter back into the region Friday night into
Saturday as the next system works closer to the area. There are
quite some model inconsistencies right now as the GFS tries to
push a front through Saturday night into Sunday and then lift it
back north, while the Euro keeps the front draped across the north
and never really pushes through. Either way we will will remain on
the active side of the weather and will continue to have
precipitation chances in through the end of the period.
12z taf discussion.
Looking at some isolated to scattered showers mainly across the
north this morning. There could be an isolated thunderstorm as
well, but would only be a possible impact to bhm, anb, and asn.
Did not include at this time as chances remain low. The main show
will come late in the afternoon and into the evening with thunderstorms and rain as
the boundary moves into the area. MVFR cigs will be around
tonight with some IFR as well. Storms may be strong/severe during
the later part of the forecast as the boundary moves in. Majority
of the rain will end after 06z in the north and 09z in the south,
with MVFR clouds building back into the area late.
Isolated to scattered showers are possible this morning as a warm
front lifts north of the area. Low clouds will be in place across
the area, with some patchy fog generally south of a Geiger to
Montgomery line. Severe thunderstorms are expected during the late
afternoon and evening hours as a line of storms moves through the
area from west to east. Light showers are possible Tuesday, before
dry conditions arrive by Wednesday. Relative humidity values will be elevated the
next couple days. Low clouds and patchy fog are possible again on
Monday night. There are no fire weather concerns.
Preliminary point temps/pops...
Gadsden 75 52 60 34 51 / 80 90 40 30 0
Anniston 75 54 64 35 53 / 70 90 40 30 0
Birmingham 78 54 61 37 56 / 80 80 40 20 0
Tuscaloosa 80 52 59 37 60 / 80 70 30 10 0
Calera 78 55 62 37 59 / 70 80 30 20 0
Auburn 76 60 68 38 58 / 40 70 30 20 0
Montgomery 81 58 67 39 62 / 40 60 30 10 0
Troy 81 62 71 41 62 / 50 50 20 10 0
dense fog advisory until 9 am CDT this morning for the following