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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 1956Z Sep 26, 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
356 PM EDT Sun Sep 26 2021

Day 1
Valid 16Z Sun Sep 26 2021 - 12Z Mon Sep 27 2021


16Z update... The rain near the stalled frontal boundary in Maine
is starting to progress east/northeast while drier air on the
backside is allowing rain to taper off quickly. The rates of 1+
inch/hour have really dropped off the past 2-3 hours therefore the
the Slight Risk area was removed from Maine. Additionally, the
western bounds of the Marginal was trimmed toward central Maine
and the southern bounds were removed from southern New England to
the coast of Maine.

For Arizona, there are a couple of mid/upper level vort maxes that
are generating areas of convection near the southeast and
east-central portions of the state that largely have had rates
less than 0.5 inch/hour. However there are a couple of very
isolated spots that have had in excess of 1-1.5 inches/hour but
are not in the area with the most favor instability. No changes
were made at this time for the Southwest.


Previous Discussion...

The approach of a trough from the eastern Great Lakes and Ohio
Valley early today will be the focus for a broad shield of rain
with embedded enhanced rainfall rates today and
tonight...especially as the trough takes a neutral and then
negative tilt towards the start of the Day 1 period.  The models
continue to focus most of the rainfall over Maine where increasing
synoptic scale forcing leads to a broad precipitation shield and
where the mesoscale forcing allows for embedded heavier rainfall
rates during the day.  The main wave should be progressive...and
the static instability looks to be modest...but there is a
quasi-stationary boundary with which the forcing aloft can biggest concern is for the cumulative effect of
several days of rain and where that might line up with any
enhanced rainfall rates.  A surge of moisture moving northward
across the eastern portion of New England appears to have enough
time to spread additional moisture ahead of the approaching trough.

...Southwest U.S./Southern Great Basin...
Maintained a Marginal Risk area with only modest modifications
based on latest guidance.  The overall pattern still focuses on a
mid- and upper-level system making its way east/northeast during
the period which supports showers and thunderstorms...especially
from mid afternoon into the evening due to steep lapse rates
resulting from maximum heating of the day in low levels and cold
temperatures aloft.  Forecast soundings still show unsaturated
sub-cloud layers which may counteract the rainfall rates
initially...and there should be areas of CIN which limits areal
coverage away from the terrain. Several days ago...the NCEP
guidance and ECMWF were in agreement that the cold temperatures
aloft would result in CAPE values approaching 2000 J per kg over
the Mogollon Rim.  With the event now being closer in time, the
models are showing the coldest temperature aloft staying farther
south that earlier forecast. This could spell lighter rainfall
rates which would work against flash flooding but may still result
in isolated run off problems in the event of multiple rounds. 


Day 2
Valid 12Z Mon Sep 27 2021 - 12Z Tue Sep 28 2021


21Z update... Very minimal adjustments made to the Marginal Risk
area that was already in effect to reflect the latest WPC QPF.


Previous Discussion...

...Southwest U.S/Southern Great Basin....
Scattered convection will persist across portions of the Southwest
while spreading into portions of southern Colorado. The latest
guidance continues to suggest an area from southwest Colorado to
north-central New Mexico could have localized higher amounts
especially in the areas of higher terrain. The models continue to
show an axis of one-quarter to two-thirds of an inch in and near
the Mogollon Rim in northern Arizona.  But many of the same
concerns expressed in the Day 1 period...where the steepest lapse
rates/best instability/highest rainfall rates set up and the
impacts of any cloud cover from the previous evening...linger in
this cycle. Maintained the Marginal for the time being.


Day 3

The Day 3 outlook will be updated by 2030Z.

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