Scientific Forecaster Discussion
fxus64 kbmx 201532
Area forecast discussion
National Weather Service Birmingham Alabama
1032 am CDT Thu Jun 20 2019
A longwave trough extended from Chicago south along the
Mississippi River valley into northeast Louisiana. Broad ridging
aloft was positioned well to our east over the western Atlantic
Basin while a general zonal flow aloft prevailed from the Southern
Plains through The Four-Corners states and into Southern
Toward the surface, low pressure was generally found in the
Central Plains along a cold front extending for western Kansas
north through South Dakota while a stationary front extended from
northern Kansas southeast through Missouri and then curved
northeast through Illinois and into Indiana and Ohio. A dryline
was analyzed south of the cold front from western Kansas through
through the far western Oklahoma Panhandle and extended south
through western Texas to the Rio Grande Valley.
Locally, the surface pressure fields were amplified by surface
reflections of the approaching upper level trough along with
several embedded shortwaves in the general zonal flow.
The 20/12z bmx sounding contained a strong subsidence inversion
around 970 mb largely due to the morning mesoscale convective
system (mcs) that moved through the area early this morning. The
cold pool/outflow temporarily stabilized the thin near-surface
layer while the profile aloft remains warmer and fairly moist. A
subsidence inversion was evident around 540 mb with drier air
higher above this layer. The overall moist profile resulted in a
precipitable water (pwat) value of 1.69 inches with much of the
moisture contained below the subsidence inversion. Storm-relative
helicity values in the 0-1 km layer were near 130 m2/s2 due to
southerly winds in the low levels gradually transitioning to
westerly flow generally above 500 mb. Wind speeds were less than
10 kts just above the surface with speeds gradually increasing to
near 40 kts just above 700 mb and further increasing to over 50
kts near and above 450 mb.
While the pre-dawn early morning mesoscale convective system has quickly departed well to
our east, a remnant outflow/cold pool from another mesoscale convective system that moved
through Mississippi early this morning was pushing east and
southeast across the forecast area this morning. A few showers and
storms developed earlier in Tuscaloosa and Jefferson counties as
a result but activity has diminished with time due to low level
stability as a result of the first mesoscale convective system.
While Richer low-level moisture was scoured out by recent
convection and cold pools, a review of surface observation trends
over the past few hours reveal that low-level moisture has been
recovering across our far southern counties and we expect this
trend to persist over the next several hours.
A fellow forecaster determined the presence of a bore wave in
eastern Mississippi earlier this morning and this feature was now
across our western counties and is expected to continue to move
east and southeast through the day today and will help to support
convective initiation late this morning and into the afternoon
hours along with the gradual steepening lapse rates as an upper
level trough moves in from the northwest.
We will be closing watching convection that develops across our
southeastern counties as this area is most supportive of severe
storms capable of producing microbursts, strong winds and
hailstones. We expect to get general shower and thunderstorm
development across much of the forecast area this afternoon with
the potential for a few strong to severe storms possible.
This evening and into the overnight hours as the upper level
trough moves across the area we will be in a northwest flow regime
aloft and there is potential that we could experience another
wave of shower and thunderstorm activity as the trough departs
before expansive deep-layer ridging fully builds into the area
from the southwest. Confidence is low due to run-to-run model
inconsistency and continued disagreement between model families.
We will continue to evaluate new data later today and will provide
forecast updates as more information becomes available.
/issued at 330 am CT/
Friday through Tuesday.
* Heat indices of 100-105f possible Saturday and Sunday.
A transition will develop through the day Friday as an upper-
level ridge begins to build across the region. This should lead to
a significant decrease in rain chances by Friday night, and with
no clear convergence zone or forcing mechanism, only very isolated
activity is expected. The ridge axis will remain just to our west
on Saturday and extend northward along the Mississippi River. Mesoscale convective system
activity is expected to ride around the periphery of the ridge
and may turn southward toward the southern Appalachians. This
complex of storms could clip our eastern counties at some point on
Saturday, but confidence is very low due to model variability and
the low predictability commonly found with this type of pattern.
The humid, Gulf airmass will linger across the southeast. As
such, heat indices are expected to increase to the 100-105 f range
on the afternoons of Sat/sun, with some locations likely approaching
criteria for a heat advisory. Will continue with the mention in
the severe weather potential statement and monitoring trends in temperatures & dewpoints in the
next day or so.
With a trough moving eastward toward the plains, the upper-level
ridge may shift eastward some for Sunday and become centered over
the forecast area. Little to no convection is expected on Sunday,
but this could change for Monday and Tuesday. There is relatively
good model support for increasing rain chances as height falls occur
with the ejection of a shortwave from the plains toward the Ohio
Valley. Increasing moisture content and low-level convergence should
result in scattered to perhaps numerous showers and storms as the
front moves across the forecast area.
12z taf discussion.
Main mesoscale convective system has moved through with a few showers and storms along a
some boundaries that are left over. Stronger storms will be
possible this afternoon so included prob30 for all sites. Most of
the activity should dissipate by midnight and MVFR ceilings may
develop across the west and south. There is a potential for
another mesoscale convective system to develop to our north and move in but models are
consistent that it may hold off until after 15z Friday.
Rain and thunderstorm chances remain high through tonight with an
active upper-level pattern. Southerly flow and tropical air will
keep relative humidity values at or above 60% percent with further
recovery after peak heating subsides. Kbdi values remain high,
but increased low-level moisture & recent rainfall will keep US
from reaching critical fire weather criteria. Drier weather with
hot afternoon temperatures are expected Friday afternoon through
Sunday with heat indices likely exceeding 100 degrees.
Preliminary point temps/pops...
Gadsden 87 68 92 70 93 / 80 40 30 20 50
Anniston 89 70 93 72 93 / 80 40 30 20 50
Birmingham 90 72 95 74 95 / 80 40 30 20 30
Tuscaloosa 92 73 95 75 96 / 60 40 30 10 20
Calera 90 71 93 72 93 / 60 50 30 20 20
Auburn 89 72 92 74 93 / 60 60 30 20 40
Montgomery 93 73 95 74 96 / 60 50 30 10 20
Troy 92 72 92 73 94 / 60 60 30 10 20