Mesoscale Precipitation Discussion 0075
NWS Weather Prediction Center College Park MD
1043 AM EST Fri Mar 01 2024

Areas affected...North-central Alabama...

Concerning...Heavy rainfall...Flash flooding possible

Valid 011545Z - 012030Z

SUMMARY...Narrow band of training thunderstorms with up to 2"/hr
rates and localized totals nearing 4" pose localized flash
flooding over the next few hours before remaining instability
wanes.

DISCUSSION...Regional RADAR loop, along with regional lightning
network denotes a long ~125 mile, but very narrow ~10-15 mile band
of thunderstorms across north-central AL slowly translating
eastward.  While the width has been narrow, the deeper layer
steering flow as remained consistent and unidirectional while
parallel to the forcing axis.  RAP along with RAOBs, observations
suggest that strong FGEN forcing between the 850-700mb layer is
providing a sharp isentropic ascent path for this persistent
vertical ascent, overcoming the limiting instability within the
region.  Weak low level ridging between an exiting 850mb low in
the Cumberland Plateau and approaching wave in east-central MS (at
the edge of core of favorable mid-level DPVA) has supported
accelerated 35-40kts of 850mb flow per BMX VWP nearly orthogonal
to this band before rapidly veering in the 700-500mb layer.  CIRA
LPW shows sfc-850mb moisture values are a tad higher than forecast
with values of 2/3rds increasing to 3/4" to increase rainfall
effectiveness.  As such, moisture flux convergence and weak
vertical instability has resulted in efficient rainfall production
with 1.5"/hr rates increasing to near 2"/hr.

Recent observations from MRMS may be a tad higher, but a few
backyard weather observations show a solid 2-3" band across  from
Lamar/Fayette county toward Etowah county, with a spot or two of
3-4" in northern Jefferson county.  This generally aligns with the
lowest FFG values in the region and as such MRMS FLASH to FFG
ratios have exceeded 200%. Even with an overestimation in MRMS,
these spot 3-4 totals are exceeding and probably resulting in a
few instances of flash flooding across that narrow axis.   As
cells move further east into northern GA, the weak instability
should further diminish and rates should reduce in turn limiting
the highest totals toward more 1-2" totals and beneficial factors
compared to further west in north-central AL. 

Gallina

ATTN...WFO...BMX...FFC...HUN...

ATTN...RFC...LMRFC...SERFC...NWC...

LAT...LON   34518536 34358498 34058497 33788527 33638596
            33548683 33458788 33598822 33918800 34138715
            34418598