Severe Thunderstorm Warning 170, NWS Birmingham, AL

Area Forecast Discussion
National Weather Service Birmingham AL
224 PM CDT Tue Jul 21 2020

.SHORT TERM...
/Updated at 1242 PM CDT Tue Jul 21 2020/

Through Wednesday.

Seeing little change in the pattern today with the western edge of
an expansive Bermuda High situated over the Southeast. Low-level
winds are relatively light, but continue to supply an east-
southeasterly stream of moisture which will fuel thunderstorm
formation as the boundary layer continues destabilizing this
afternoon. The available moisture has increased since yesterday
which should allow for higher coverage of scattered convection,
especially as boundary interactions begin to occur, but still kept
max PoPs around 50% due to the lack of organized forcing. On the
other hand, some moistening of the mid-levels occurred overnight
which has lowered microburst parameters by a small amount, although
water loading may come into play in this case with PWATs ~1.8-1.9
inches. As temps rise into the mid 90s with dewpoints in the lower
70s in addition to dry mid-level air advecting northward from south
Alabama, the atmosphere will certainly be unstable enough to sustain
a few strong thunderstorms capable of producing gusty winds and
frequent lightning. Therefore, I will leave mention of isolated
damaging wind gusts up to 60 mph in the HWO text.

The combination of high temps and moist surface conditions continues
to produce heat indices ranging from 100-105 degrees, so take it
easy while spending time outdoors. The ridge persists tomorrow and
we'll see a near repeat in the forecast. Highest rain chances will
be focused across the northern half of the forecast area where the
highest available moisture will be present. And again... highs will
be in the mid 90s.

86

.LONG TERM...
/Updated at 0224 PM CDT Tue Jul 21 2020/

Thursday through Monday.

Overview.

Few adjustments made to the ongoing long term portion of the
forecast as ridging remains the dominant factor in our local
conditions through the period.

Features.

Upper-level ridging will remain over the area through the day
Thursday and will persist into early next week despite it
gradually migrating west Friday through Monday, becoming centered
more over the Central Plains region.

Broad mid-level ridging will remain over the area with the center
of the ridging becoming centered to our northwest over the Mid
Mississippi River Valley Region from Friday into Saturday, then
expanding southward to encompass much of the Central Plains on
Sunday.

Lower-level ridging will be centered off of the Southeast
Atlantic coast while a tropical disturbance moves west across the
Central Gulf of Mexico toward Texas and Mexico from Thursday
through Sunday. The associated easterly flow off the surface will
help to increase moisture levels from the Central Gulf Coast
region and further southwest over portions of Louisiana and Texas
with a more subtle increase in moisture expected this far north.

As has been the case over the past few weeks now, above average
high temperatures are expected while isolated to scattered showers
and storms will provide some temporary heat relief. Kept PoPs
generally in the 50 percent range or lower due to uncertainty in
location that convection will develop each day. With the lack of
synoptic-scale forcing, convection will mostly be driven by
mesoscale processes with the placement and movement of boundaries
along with differential heating from cloud edges and terrain
features.

High temperatures will remain in the 90s through the period and
this combined with dew point values in the low to mid 70s will
create heat index values peaking out in the 100-105 range.
Overnights will be quite humid and warm with lows in the low to
mid 70s through the period as a tropical airmass in the lower
levels prevail due to lower-level ridging remaining to our east
which promotes a southeast low-level flow across the region.

Forecast.

Expect isolated showers and storms Thursday and Friday with highs
ranging from the low to mid 90s while lows will range from the
low to mid 70s.

Some increase in moisture aloft is expected for the latter
portion of the long-term period which will permit isolated to
scattered shower and storm activity across the area Saturday
through Monday. Generally, the better chances look to be across
the east and south but all areas should have at least isolated
convective activity. Highs will remain in the low to mid 90s each
day with lows in the low to mid 70s overnight.

05

&&

.AVIATION...
18Z TAF Discussion.

The western periphery of the subtropical ridge remains expanded over
Central AL producing light east-southeasterly winds. Moisture has
increased today which should allow for a higher coverage of diurnal
thunderstorms. Greatest period of potential flight impacts will
generally be from 18-00Z with activity diminishing thereafter, and
highest chances will be near the northern terminals with coverage
maximized by late afternoon. A few scattered thunderstorms are
already beginning to impact TAF sites this afternoon. The typical
uncertainties regarding the disorganized nature of summertime
convection does lower my confidence on targeting any one TAF site
for potential impacts, but did include a TEMPO for TCL and EET based
on the thunderstorm activity currently on radar. Added VCTS in at
the other sites, but will amend as needed throughout the afternoon.
Otherwise, may see some areas of patchy fog again early tomorrow
morning, but not expecting any major flight impacts, so did not
include in this TAF cycle.

86

&&

.FIRE WEATHER...

Afternoon minimum RH values should remain near or above 40
percent through Thursday and above 45 percent Friday through
Monday. Isolated to scattered showers and thunderstorms will
remain in the forecast through the period with best chances during
the afternoon and into the early evening hours. Some localized
areas of patchy fog may develop overnight in areas that receive
heavy rain earlier that day. Surface and transport winds are
expected to remain fairly light out of the southeast generally.
Heat indices each afternoon are expected to peak in the 100-105
range.

05

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     72  93  71  95  72 /  20  50  10  40  10
Anniston    73  94  72  95  72 /  20  40  10  40  10
Birmingham  75  95  74  96  74 /  20  50  10  30  10
Tuscaloosa  75  96  74  96  74 /  20  40  10  30  10
Calera      73  94  73  95  72 /  20  40  10  30  10
Auburn      73  93  73  93  72 /  20  40  10  30  10
Montgomery  74  96  74  96  74 /  20  30  10  30  10
Troy        73  95  73  96  73 /  20  30  10  30  10

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
None.

&&

Hazardous Weather Outlook
National Weather Service Birmingham AL
241 AM CDT Tue Jul 21 2020

ALZ011>015-017>050-221045-
Marion-Lamar-Fayette-Winston-Walker-Blount-Etowah-Calhoun-Cherokee-
Cleburne-Pickens-Tuscaloosa-Jefferson-Shelby-St. Clair-Talladega-
Clay-Randolph-Sumter-Greene-Hale-Perry-Bibb-Chilton-Coosa-Tallapoosa-
Chambers-Marengo-Dallas-Autauga-Lowndes-Elmore-Montgomery-Macon-
Bullock-Lee-Russell-Pike-Barbour-
241 AM CDT Tue Jul 21 2020

This Hazardous Weather Outlook is for the counties served by the
National Weather Service office in Birmingham.

.DAY ONE...Outlook through Tonight.

A few strong to severe thunderstorms may occur this afternoon with
isolated damaging wind gusts up to 60 mph.

Heat indices are forecast to reach 100-105 degrees this afternoon.
Take extra precautions if spending time outdoors.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday.

Heat indices are forecast to reach 100-105 degrees each afternoon
through Monday. Take extra precautions if spending time outdoors.

.SPOTTER INFORMATION STATEMENT...

Activation of storm spotters and emergency management is not
expected at this time.

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