Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
315 AM EDT Mon Aug 30 2021
Valid 12Z Thu Sep 02 2021 - 12Z Mon Sep 06 2021
...Ida Heavy Rainfall/Flooding Threat to spread from the
Mid-Atlantic to Southern New England Thursday...
...Nora lead moisture to fuel Southwest U.S./Rockies Heavy Rain
threat into Thursday...
...Emerging late week locally heavy convective rainfall pattern
for the Northern Plains/Upper Midwest...
...Guidance Evaluation and Preferences...
The WPC medium range product suite was primarily derived from a
composite blend of reasonably well clustered but smoothed guidance
from the GEFS/ECMWF ensemble means, the NBM and with infusion with
the latest implications from NHC tropical tracks for Nora and Ida.
Individual models are beginning to offer some more common trends
into medium range time scales, but still differ enough from run to
run to favor the more ensemble mean forecast approach that offers
better continuity in a pattern with near normal predictability.
...Pattern Overview with Weather/Hazard Highlights...
By Thursday and into Friday, Ida as a still well organized
post/extra tropical cyclone is forecast by the NHC to lift from
the Mid-Atlantic to offshore southern New England with uncertain
interaction from northern stream energies. This track would bring
a heavy rainfall threat fueled by deep wrapping moisture in a
favorable right entrance region of a potent upper jet. Cell
training and terrain will significantly enhance the runoff/flood
threat within a well defined precipitation axis.
The latest track guidance for Nora brings it northward along the
western Mexico coast and into Northwest Mexico where it will
weaken rather quickly as it interacts with the terrain. Leading
deep tropical moisture ahead of Nora will fuel a lingering
Southwest U.S./Rockies heavy rain threat into Thursday.
Elsewhere, ample northern stream upper troughing will meanwhile
dig a wavy and modestly unsettling/cooling front through the West
and downstream across the north-central states. Upper dynamics and
frontal convergence/progression should act to focus some locally
heavy convection/rainfall as enhanced by embedded impulses/frontal
waves as fueled by instability and frontal moisture pooling,
including some monsoonal influx Thursday and Friday.
Schichtel
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml