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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 0828Z Sep 25, 2021)
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Sat Sep 25 2021

Day 1
Valid 12Z Sat Sep 25 2021 - 12Z Sun Sep 26 2021


Moisture continued to be directed from the North Atlantic towards
the eastern portion of Maine in a narrow band as of early
Saturday.  Given good agreement with the overall pattern by the
various is not much reason to stray from their
consensus solutions which shows a relative lull in activity before
the approach of the next shortwave trough results in renewed
moisture transport into the region from the Atlantic,
strengthening upper level jet dynamics and redevelopment of
organized rainfall later tonight/early Sunday morning.  Given the
lack of any significant static instability...but frontogenesis
becoming organized through a deeper layer in the 26/00Z to 26/06Z expecting that there will be a broad shield of rain
with bands of locally heavier rates embedded within area
especially over eastern Massachusetts northward into western
Maine. Given the amount of rain over the past couple of
days...still think there could be problems due to run-off. 
However, there was not any significant change which would require
an upgrade to a Slight Risk at this point.

...Far Southwest...
Moisture has been drawn into the Southwest U.S. over the past 24
hours by an area of low pressure located to the west of the Baja
peninsula.  As the system begins moving eastward and makes its way
across the northern Baja peninsula later tonight/early Sunday
morning, there should be some showers and thunderstorms during the
latter half of the period forming over southern/central Arizona
into western New Mexico which can tap the airmass with
precipitable water values ranging from 0.75 to 1.00 inches and
produce locally heavy rainfall.

Few changes were needed to yesterday's Day 2 ERO as it propagates
into the Day 1 period.  The expansion of the Marginal Risk still
looks okay and covers th spread shown by the operational suite of
guidance as well as the spaghetti plots from the high resolution
models and the HREF probabilities. 

...South Florida...
Introduced a Marginal Risk area over the far southern Florida
Peninsula again given the model signals for slow-moving convection
to form in an airmass with precipitable water values approaching 2
inches later this afternoon and evening.  The high resolution
guidance is showing spotty 2 inch per hour rates possible along
and near the urbanized corridor during the late
afternoon...although the forcing will be weak so confidence in
exact evolution is limited.


Day 2
Valid 12Z Sun Sep 26 2021 - 12Z Mon Sep 27 2021


The approach of a synoptic scale wave towards the end of Day 1
will still in a position to focus and support a broad shield of
rainfall mainly confined to Maine...with enough mesoscale forcing
in the form of frontogenesis to keep over New England to result in
bands of enhanced rainfall rates.  The entire area should
gradually translate eastward during the day with rain tapering off
from west to east.  With rainfall amounts across Maine expected to
be 1 to 1.5 inches...with locally higher amounts...the concern
remains more for the cumulative effect of any heavier amounts on
top of the rainfall received in the preceding days.

...Southwest U.S...
Maintained a Marginal Risk area in similar proximity to the area
depicted on Day 1 as a closed low begins to move northeast across
southern Arizona.  Both the GFS and ECMWF both move the mid- and
upper-level system eastward and into the far Southwest U.S. while
any low-level circulation gets left behind off the Baja
peninsula...and the guidance is in general agreement that the
system will begin to weaken it it makes its way across Arizona
towards Mexico.  The ECMWF was slower than the GFS in weakening
the system...but both models agree that showers and thunderstorms
will accompany the low. Thinking is that cold temperatures aloft
will result in enough instability to for cells to produce locally
heavy rainfall rates from the best heating of the afternoon into
the evening.  Used a mixture of GFS and ECMWF instability guidance
to defined the boundary of the Marginal Risk in Arizona and
western New Mexico.


Day 3
Valid 12Z Mon Sep 27 2021 - 12Z Tue Sep 28 2021


...Southwest U.S/Southern Great Basin....
A continuation of showers and thunderstorms is
expected...especially during the latter part of the day on Monday
into the per-dawn hours Tuesday a deep layer low
makes its way slowly eastward across the southern Great Basin.  By
this point, the NCEP guidance and ECMWF agree that the mid- and
upper-low will be well-separated from any low level circulation
and should be in a weakening phase. Even so, colder temperatures
aloft could result in enough instability to support locally heavy
rainfall at the time of maximum daytime heating. Model guidance is
split between the ECMWF camp which holds on to a stronger, better
defined upper level system with colder temperatures at the core
compared to the GFS which was quicker to allow the upper system to
weaken.  Focused the Marginal Risk area along/near the Mogollon
Rim northeastward towards the 4 Corners area where the best
instability overlapped with the best/deepest moisture.


Day 1 threat area:
Day 2 threat area:
Day 3 threat area: