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000
FXUS02 KWBC 201939
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
338 PM EDT Tue Aug 20 2019

Valid 12Z Fri Aug 23 2019 - 12Z Tue Aug 27 2019

...Overview and Guidance/Predictability Assessment...

16Z Update:

Minor changes needed from previous forecast issuance for this
cycle. The main notable model guidance change toward the latter
portions of the forecast period where the 00Z ECMWF trended toward
ridging off the New England coast (12Z run yesterday showed a
closed low forming Sunday), which is more aligned wit the rest of
the deterministic and ensemble guidance. Thus, more inclusion of
the ECMWF and its ensemble mean was included for Day 5-7.
Otherwise, fairly typical summertime pattern expected across the
CONUS with a couple shortwaves tracking across the northern tier
of the U.S.

---previous discussion---

An upper trough extending from eastern Canada into the
northeastern U.S. as of early Fri will progress eastward with time
while fast flow arriving from the North Pacific should develop
broad mean troughing across the northern tier states from the
weekend into early next week.  Upper level ridges should prevail
over the southern Rockies/Four Corners region and over the eastern
Pacific (the latter ridge possibly extending into the western
U.S.).  Guidance suggests a relative weakness aloft will persist
over the western Gulf Coast/Lower Mississippi Valley.  The most
prominent uncertainties involve specifics of the northeastern
trough and combined details of northern tier flow/upstream ridge.

Among the 12-18Z guidance one forecast consideration was to
downplay some 12Z ECMWF details, as it and its mean were more
amplified than other solutions for the shortwave reaching the
northern High Plains by Fri while the operational run developed a
closed low just off the New England coast by Sun.  The previous
ECMWF run also formed a closed low near New England.  This
scenario was within the full ensemble spread but so far confidence
was too low to depict in a single deterministic forecast.
Interestingly the new 00Z UKMET/CMC use farther west shortwave
energy to close (CMC) or nearly close (UKMET) an upper low that
crosses New England on Sat with faster timing than the ECMWF by
Sun.  At this time the main idea is that it could take some
additional time for troughing to cross New England after the core
of the initial upper trough crosses the Canadian Maritimes.
Returning to the northern Plains energy, new 00Z runs still
suggest the 12Z ECMWF/ECMWF mean could be on the strong side.

By the latter half of the period guidance is still showing
considerable sensitivity regarding eastern Pacific/western U.S.
upper ridge details and effects on northern tier troughing.
Through the 12-18Z cycles the ECMWF/CMC and their means have been
fairly consistent (minus a modest slower trend) for at least a
couple days in showing a strong enough eastern Pacific into
western U.S. ridge to yield a downstream trough axis over the
northern Plains/Upper Midwest by next Mon-Tue.  Recent GEFS means
have been tending to show a farther westward ridge/trough while
GFS runs have varied between the two camps.  The 18Z GEFS mean had
elements of both.  The positive height anomaly center seen in
multi-day means late in the period have generally been favoring
the ECMWF/CMC cluster but latest means show potential for the
anomaly center to be just a little farther west--and according to
teleconnections possibly enough to change the character of the
trough.  So preference leaned about two-thirds toward the
ECMWF/ECMWF mean cluster relative to the 18Z GEFS mean but with
somewhat lesser confidence than was previously the case.  New 00Z
CMC/ECMWF runs that have adjusted westward further lower
confidence in a specific solution.


...Weather Highlights/Threats...

Compared to yesterday the latest guidance is showing a stronger
signal for heavy rain potential over parts of the northern
Plains/Upper Midwest and possibly into the Great Lakes.  As strong
Pacific flow carves out a mean trough over the northern tier
states, leading height falls aloft and associated cold front(s)
plus a leading warm front may progress along slowly enough to
allow for some training/repeating convection.  However it will
take time to resolve important lower predictability shortwave
details and significant changes in the forecast are possible.
Another area of interest for possible heavy rainfall will extend
from the west-central Gulf Coast through the Southeast/southern
Mid-Atlantic.  A wavy front settling into the Southeast will
provide a multi-day focus for rainfall while energy aloft/moisture
within or east of the general upper weakness over the western Gulf
Coast/Lower Mississippi Valley may help to generate one or more
episodes of enhanced rainfall near the Gulf Coast.  Some of this
moisture could end up interacting with the Southeast front.

The primary area of above normal temperatures should extend from
southern Oregon and northern-central California into the southern
High Plains with some pockets of plus 10F or greater anomalies for
highs and/or morning lows.  The northern High Plains may see highs
at least 5-10F below normal this Fri and then again early next
week.  Forecast confidence over parts of the West and Plains
decreases considerably by next Mon-Tue though.  Southern Canada
high pressure building into New England and clouds/rain with the
front to its south should provide near to below normal highs over
the eastern U.S. for multiple days.

Rausch


Hazards:
- Heavy rain across the central Gulf Coast and much of the
Mid-Atlantic, as well as portions of
North and South Dakota, Fri-Sat, Aug 23-Aug 24.
- Heavy rain from across the central Gulf Coast to the interior
portions of the Southeast, Sun-Mon,
Aug 25-Aug 26.
- Heavy rain across portions of the Southern Appalachians,
Fri-Mon, Aug 23-Aug 26.
- Heavy rain from across much of the Upper Midwest, Sun-Tue, Aug
25-Aug 27.
- Flooding possible across portions of the Central Plains and the
Middle Mississippi Valley.
- Flooding occurring or imminent across portions of the Central
Plains, the Southeast, and the
Northern Plains.

Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards 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

$$




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