Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
446 PM EDT Mon Oct 02 2023

Day 1
Valid 16Z Mon Oct 02 2023 - 12Z Tue Oct 03 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN HIGH PLAINS...

...16z Update...

...Southern High Plains...

Only a minor expansion of the northwest edge of the Slight Risk
area was needed for this update, as the 12z HREF remains in good
agreement concerning convective development across the region this
afternoon. The HREF 40-km neighborhood probabilities for 2"
exceedance generally range from 20-40% across the Slight Risk
area, and probabilities for 3" exceedance are maximized at the
southern end of the area (where FFGs are considerably higher).

...Florida...

No changes needed for the Marginal Risk area, as the 12z HREF
continues to indicate relatively high odds (up to 35%) for
localized 5" exceedance over the course of the day, maximized near
the relatively sensitive Miami metro.

...Texas Coast...

The Marginal Risk area was adjusted a bit based on the new
guidance, but this area also remains on track for localized 5"
exceedance risk through the day (per 12z HREF 40-km neighborhood
probabilities of 10-30%).


Churchill


...Previous Discussion...

...Southern High Plains...

An upper trough propagating through the West will trigger showers
and thunderstorms across the High Plains today. The Southern High
Plains, in particular, will face the threat of excessive rainfall
and flash flooding. PWATs will be between 1-1.5" with decent low
level southerly flow. The instability this afternoon will be over
1000J/Kg meaning storms that initiate could produce efficient rain
rates of over 0.5"/hr over recently saturated soils. The Slight
Risk area, in coordination with ABQ, was expanded northwestward
since the last issuance to account for the especially vulnerable
Hermits Peak Calf Canyon burn scar. The Slight's southwestern
extent was maintained to account for a consistent GFS and HREF
signal for another round of late night/early morning convection
near Pecos and Monahans in west Texas. HREF probabilities are
hitting on high confidence of over 3" in that area tonight.

...Florida...

The Marginal Risk area from the last issuance was curtailed to
encompass only southeastern Florida surrounding the eastern Keys
and the Miami metro. A slow moving surface front is expected to
continue its southward trajectory today. The main threat of flash
flooding will be over the urban corridor where there's moderate
confidence of HREF probabilities reaching over 3".

...Texas Coast...

There's been a persistent signal for some heavy rain to occur
along a convergence zone within southern Gulf Coast of Texas
today. Ample instability around 1500-2000J/Kg and a modest low
level jet will support a few intense rounds of convection this
afternoon. HREF probabilities suggest moderate confidence of over
3" of rainfall. Urban areas such as Corpus Christi will be
susceptible to flash flooding.


Kebede



Day 2
Valid 12Z Tue Oct 03 2023 - 12Z Wed Oct 04 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE GREAT PLAINS AND THE GULF COAST OF TEXAS...

...21z Update...

Minimal changes were needed for this updated, as guidance remains
in fairly good agreement concerning a broad area of rainfall
across the central portion of the country. Signals remain
insufficient for a targeted Slight introduction, and any excessive
rainfall capable of flash flooding is expected to be localized and
difficult to pinpoint.

Along the Texas coast, the Marginal Risk was expanded a bit based
on the 12z HREF guidance. Conversely, the signals across the
Florida Atlantic Coast did indeed continue a declining trend, so
the Marginal Risk was removed for this update.

Churchill


...Previous Discussion...

...Great Plains...

A broad Marginal Risk of excessive rainfall is in place across the
Great Plains and parts of the Upper Midwest on Tuesday as moisture
streams out ahead of the deepening upper trough emerging over the
High Plains. Showers and thunderstorms will focus around a cold
front extending down across the Plains with the greatest chance
for excessive rainfall occurring closest to the greatest
instability from the Southern Plains up into northern portions of
the Central Plains. Guidance has consistently signaled this, but
has lacked a strong signal for especially high rain rates to
overcome relatively dry soils at this time.

...Texas Coast...

Another Marginal Risk area was introduced for this issuance to
account for another round of convection on Tuesday along a coastal
convergence zone. Operational guidance has picked up a signal for
this and the similarities with today's setup of low level
convergence within an unstable environment and a modest low level
jet support the possibility of excessive rainfall leading to
isolated flash flooding.

...Florida...

Recent rainfall has made much of Florida's Atlantic Coast
susceptible to flash flooding, despite the operational guidance
keeping the heaviest QPF well offshore. The confluence of these
two factors should support a low confidence Marginal Risk area
along the coast for now. If operational QPF trends continue on a
downward trend then the Marginal Risk area might be removed
altogether on the next issuance.


Kebede

Day 3

The Day 3 outlook will be updated by 2030Z.


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt