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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1559Z Jul 02, 2020)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Thu Jul 02 2020

Valid 12Z Sun Jul 05 2020 - 12Z Thu Jul 09 2020

1600 UTC Update...

Overall, models/ensembles continued to show relatively good
agreement/consistency through the extended forecast period,
especially at larger scales. Thus, no major changes were made to
the ongoing forecast with this update. The forecast was updated to
incorporate some of the more recent model guidance, including the
06Z GFS/00Z ECMWF. A heavily deterministic blend was used during
days 3-4 (Sun-Mon), with a gradual trend toward heavier weight
placed on the 00Z ECENS/06Z GEFS ensemble means during days 5-7
(Tue-Thu).

Ryan


Previous Discussion (issued at 0650 UTC)...


...Overview and Guidance Assessment...

The operational models and ensemble means maintain good agreement
and consistency for the large scale pattern evolution.  Expect
some amplification of a persistent West Coast/eastern Pacific
upper trough during the period while the center of a ridge
initially over the Southwest/southern Rockies and northern Mexico
shifts back to the southern Rockies and southern half of the
Plains.  Downstream a central Canada ridge and eastern
Canada/Northeast U.S. mean trough should give way to increasingly
progressive nearly zonal flow.  A diffuse upper weakness/trough
over the southeastern quadrant of the lower 48 this weekend will
trend steadily weaker and/or shear out with time.

Within this background pattern the forecast challenges for
embedded details are similar to past days and fairly low in
predictability due to their relatively small scale--impulses
within and ejecting from the West Coast trough as well as
specifics within the initial Southeast upper trough--or the
evolution toward progressive and low amplitude mean flow as is
expected across the northern tier U.S./southern Canada.  Thus it
is not surprising that continuity has been less than optimal for
frontal position/timing from the Northern Plains through the
Northeast/Great Lakes.  Over the past day a consensus blend has
held up fairly well for low pressure from the Southeast to East
Coast and possibly western Atlantic but there is still a fair
amount of spread for where a low center may be at a particular
time.

The agreeable large scale pattern and low confidence in specifics
favor maintaining a forecast approach that incorporates the 18Z
GFS and other models from the 12Z cycle early in the period
followed by a gradual transition toward a blend of models and 18Z
GEFS/12Z ECMWF means.  Later in the period ECMWF input included
both runs up to the 12Z cycle due to timing differences with an
impulse expected to eject from the West Coast trough and produce a
northern tier wave by day 7 Thu.  Regarding this feature the 12Z
ECMWF/ECMWF mean were on the leading side of the spread while
12Z/18Z GFS runs seemed a bit slow to eject it.  Minor edits
yielded a compromise timing.  The new 00Z GFS compares better to
other solutions, showing better definition of energy dropping into
the core of the upper trough off Vancouver Island by next Thu and
in turn faster ejection of the shortwave of interest.


...Weather/Hazard Highlights...

The Northern Plains and possibly into the Upper Mississippi
Valley, along with an area from the central/eastern Gulf Coast to
the Southeast Coast will be the two regions in focus for
potentially heavy rainfall.  Frontal specifics over the northern
tier have varied from day to day but there has been more
consistency from a multi-day rainfall perspective due to initially
slow progression of a leading Northern Plains front and then the
trailing part stalling as it awaits ejection of shortwave energy
from the West Coast upper trough.  Confidence remains low for
specifics of Southeast rainfall due to the small scale of features
embedded within the overall weakness aloft and possible dependence
on mesoscale influences on rain/thunderstorms.  Less organized
rainfall of varying intensity will be possible over parts of the
Rockies and Central Plains as well as the Northeast/Mid-Atlantic. 

Continue to expect the highest temperature anomalies to be
centered over the Great Lakes during the period with some daytime
highs up to 10-15F above normal.  A broader area covering much of
the northeast quadrant of the lower 48 will see slightly less
extreme anomalies.  New England will start closer to normal but
trend warmer by midweek as flow aloft flattens.  Most of southern
Florida and the Southwest will be moderately above normal for high
temperatures but heat index values may be sufficiently high to
represent excessive heat potential.  Somewhat below normal highs
will be possible near the West Coast mean trough aloft, over the
northern High Plains during the first half of the period (behind
the leading Northern Plains front), and with clouds/rainfall over
parts of the Southeast.

Rausch


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