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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1602Z Jul 19, 2018)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1202 PM EDT Thu Jul 19 2018

Valid 12Z Sun Jul 22 2018 - 12Z Thu Jul 26 2018

...Heat Wave to Intensify Over the Southwest States Next Week...
...Heavy Rain / Flash Flooding Expected Over the Eastern U.S. Next
Week...

...Overview and Guidance Evaluation/Preferences...

Troughs are of medium to long wavelength and fairly slow to evolve
given the season. They are also relatively well defined, with one
closed low center expected in the east early next week, eventually
replaced by another well defined upstream trough digging into the
Great Lakes by Wed/Thu. So the main storm track is up and over the
Southern Plains Ridge, and this mean trough in the
northern/eastern U.S. has ample access to a tropical moisture
plume that backs in off the Atlantic into the eastern quarter of
the nation. Meanwhile beneath the ridge, the heat wave in the
Southern Plains this weekend then expands westward as the ridge
becomes repositioned, with the 594 decameter 500-mb height
covering a very broad expanse by Tuesday.  In some places like
northern Mexico and southern AZ/CA, where monsoon season is more
commonly in full swing by now, these heights peak at 3.5 standard
deviations above climatology. Standard deviations of +2.0 extend
as far north as San Francisco and Salt Lake City.

Much of this medium range forecast is of high confidence during
this cycle, with strong agreement / clustering among the models
regarding these well defined and slowly evolving features. Most of
the more noticeable model differences are across the very high
latitudes, whereas the forecast over the CONUS shows less spread.
We therefore began with a simple GFS/ECMWF blend, but began to
work in more of the GEFS and ECMWF Ensemble Mean. The operational
00z and 06z GFS were the first to be dropped from the blend as
they were quite aggressive in digging the closed low southwestward
along the Gulf Coast D4-5. The operational ECMWF saw more ensemble
support in this regard, and perhaps the GFS solutions is being
driven more by convective feedback onto the synoptics. By Days 5-7
we were using roughly a 40/40/20 blend of GEFS mean / EC Mean /
Operational ECMWF.

...Weather Highlights/Threats...

Significant hazards expected over broad areas of the country the
next seven days. Heat stress will be a major story in the West.
Forecast highs are in the upper 100 and teens (110-119) in the
desert areas (including Phoenix, Vegas) for several consecutive
days. Low temperatures near or even above 80 deg for many areas
will amplify the potentially hazardous impacts of the heat. The
potential arrival of a cold front across the southern plains by
next Tue-Wed could bring some slight relief there, but the heat in
the Southwest looks to be a lengthy and potentially dangerous
event.

Flash flooding and perhaps even longer term, larger scale flooding
(e.g., river flooding) will be possible in broad swaths of the
eastern states. The closed low dropping through the OH/TN Valleys
toward the Gulf Coast anchors a well defined trough, with
precipitable water of 2.0 inches or greater forming an axis that
overlaps with many mountainous areas in the Appalachians and New
England. These areas are prone to flash flooding, and the
combination of the synoptic support along with seasonably strong
sun angle will likely yield pockets of instability sufficient to
support an off and on heavy rainfall threat.

Burke/Ryan


WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found at:

http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
http://www.wpc.ncep.noaa.gov/heat_index.shtml