Scientific Forecaster Discussion
fxus64 kbmx 211856
Area forecast discussion
National Weather Service Birmingham Alabama
1256 PM CST Thu Nov 21 2019
for 18z aviation.
/updated at 0353 am CST Thu Nov 21 2019/
today and tonight.
This am at the surface, a north/south oriented high pressure ridge
stretches across the eastern third of conus. To the northwest a
developing low pressure system and associated front are moving
southeast across the central U.S. Along with a large ridge sinking
southward behind it into the upper rockies and Dakotas. This front
is expected to continue to the southeast through tonight pushing
through the Great Lakes, mid Mississippi River valley and North
Texas by sunrise Friday. At the same time in the upper levels, a
somewhat zonal/Flat Ridge is in place over much of eastern conus.
To the west lies an upper trough across central Canada extending
southwestward into a Southern California/Nevada closed low. The
upper trough will move east toward the Great Lakes across the
northern U.S. Leaving the cut off closed low lagging behind over
The Rockies and a flattened zonal flow over Alabama.
In response to the synoptic pattern, our surface winds will begin to
turn today to a more southeast/south direction ahead of our
approaching system. A slow creep upward of our dew points from west
to east will result today as moisture filters back into the area off
of the Gulf. We currently are very dry with a precipitable water of
40 on the 11/21 00z bmx sounding. It will take a little while
for sufficient moisture return to occur. Guidance indicates barely
1.0+ by sunrise Friday. Although no precipitation is expected for
today, we will see a continued moderating of our airmass with
higher temperatures for today and milder for tonight. For the most
part, rain should hold off until later during the day Friday as
our overall moisture continues to increase. However, I do have
some low pops in the far northern counties for tonight as the
/updated at 0353 am CST Thu Nov 21 2019/
Friday through Saturday.
A broad upper low will move eastward from Colorado to Missouri
Friday/Friday night, with a neutrally to positively tilted trough
along its southern flank. It will interact and possibly phase with a
northern stream shortwave trough Saturday/Saturday night, probably
opening up into an open wave while taking on a negative tilt as it
moves to the mid-Atlantic region Saturday night. At the surface, a
cold front associated with low pressure over Quebec will sag
southward into northern Mississippi and the Alabama/Tennessee border on Friday
before stalling. A developing wave of low pressure over the arklatex
will cause it to take on characteristics of a warm front and sharpen
late Friday afternoon and Friday night. The new surface low will
deepen as it lifts northeastward across northern Mississippi Friday
night, reaching middle Tennessee by Saturday morning and western PA
by Saturday evening, with the trailing cold front crossing central
Alabama on Saturday.
A broad area of moist deep layer southwesterly flow will be present
along and ahead of the front on Friday, with several embedded
shortwaves. Pwats will increase to around 1.5 to 1.6 inches
Friday/Friday night. Moist isentropic lift will result in several
waves of showers moving through Friday/Friday evening across the
northwest half of the area as the column saturates with the front
also providing some forcing. This will also result in a tricky high
temperature forecast with increased cloud cover. Southerly flow and
rain-free conditions will result in highs continuing to be above
guidance in the south, while highs across the north will be
dependent on how quickly the rain moves in, and a secondary warm
front may set up in between. An additional area of showers will set
up along the cold front which moves in late Friday night and
Saturday. An isolated embedded thunderstorm is possible late Friday
night in the west with some weak, primarily elevated instability.
Dew points only in the 60-62f range and lack of stronger Theta-E
advection, combined with expected cloudcover and precipitation
should prevent any strong or severe surface-based activity late
Dew points may creep up to around 65f on Saturday in our far
southern counties, which may allow some weak surface-based
instability to develop, though the consensus of guidance keeps cape
values still only around 200 j/kg or less. Forecast soundings also
suggest hodographs will straighten as low-level flow veers with time
even as instability begins to increase, and profiles remain
saturated. If an overlap between enough instability and curved
hodographs could develop, then there would be a non-zero threat of a
brief tornado on Saturday, but this seems unlikely at this time,
though some gusty winds will be possible. Rainfall amounts look to
be around 0.5 to 1.5 inches, with locally higher amounts in the far
northwest, so with a lack of saturated conditions any minor flooding
should be limited to poor drainage areas. Conditions mainly dry out
Saturday night, but can't rule out some drizzle across the far north
as some wrap-around low-level moisture rotates through.
Sunday through Thursday.
Rain-free and cooler conditions are expected Sunday, as a few
moisture-starved waves move through in northwest flow aloft.
Southerly winds on Monday will signal a warming trend. Meanwhile a
strong North Pacific jet will carve out broad troughing from the
northwestern to the north-central Continental U.S.. models have been trending
less amplified with a shortwave and strong mid-level speed Max
rounding the base of this trough Tuesday/Tuesday night, but vary
regarding how much less amplified. There is still ensemble support
for a surface low lifting northeast from the Southern Plains to the
Great Lakes, with high rain chances and possible breezy conditions
for central Alabama with the cold front passage Tuesday night.
Moisture return and instability within the narrow warm sector is
expected to be somewhat limited, and heights remain neutral/rising
with time. Therefore any potential severe threat remains very
marginal at this time. Strengthening ridging over the Gulf of Mexico
is expected through the end of the period downstream of West Coast
troughing. A very early look at the Thanksgiving forecast suggests a
warm front lifting northward across West Alabama while a cad wedge
tries to build in from the east, suggesting varied temperatures
across the area and some light rain chances especially the further
north you go, but details remain murky at this time.
18z taf discussion.
VFR conditions are expected through the rest of the day and
evening. Southerly winds will bring plenty of mid level moisture
overnight to reduce ceilings to MVFR at each taf site. Light
showers are possible beginning Friday late morning...though
confidence is too low in exact timing and coverage to mention in
any taf other than ktcl where confidence is a little bit higher.
Even then...only mentioned vcsh for ktcl for now.
Dry conditions are expected through this afternoon with high
pressure in place and mild temperatures. The chance for wetting
rains returns tonight and lasts through Saturday thanks to a
frontal system that will move across central Alabama. Minimum
relative humidity values will go into the mid 30s this afternoon,
but will be higher Friday into the weekend as rain chances return.
Preliminary point temps/pops...
Gadsden 46 65 57 65 38 / 20 80 90 90 10
Anniston 48 68 58 67 41 / 10 60 90 90 10
Birmingham 51 68 60 65 41 / 10 70 90 90 10
Tuscaloosa 52 71 61 65 39 / 10 70 90 80 10
Calera 49 70 60 65 40 / 10 50 90 90 10
Auburn 48 68 58 69 43 / 0 10 50 90 20
Montgomery 49 76 60 73 44 / 0 20 60 90 10
Troy 48 75 59 74 44 / 0 10 30 90 20