Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1144 AM EDT Wed Aug 21 2019
Valid 12Z Sat Aug 24 2019 - 12Z Wed Aug 28 2019
...Overview and Guidance/Predictability Assessment...
The latest model guidance continues to suggest the establishment
of a broad mid/upper level trough over the northern CONUS,
sandwiched between an eastern Pacific ridge that builds into the
northeastern Pacific through the period and another ridge across
eastern Canada. Individual, repeating shortwaves will drop through
the mean flow across the northern tier of the U.S. during this
time frame. Model guidance is in reasonably good agreement for Day
3 and 4 with above average predictability and less spread noted
within the model envelope. Across the southern latitudes, ridging
will hang on across the southwest U.S. while a weak shortwave will
hover over the western Gulf into the Lower Mississippi River
Valley.
Model guidance diverges late in the weekend and early next week
with respect to how the eastern U.S. upper level ridge evolves and
whether a closed low forms off the New England coast. The past
several runs of the ECMWF have been the most aggressive with this
idea, forming the closed low closest to the coast. Meanwhile, the
latest GFS and CMC have a similar idea, but further to the east
and more progressive as upstream shortwaves approach. The
preference was for more inclusion/weight toward the ECMWF and its
ensemble mean, especially by Day 6-7. Additionally, models diverge
on a potential low forming off the southeastern U.S. by Day 6-7
associated with an area of disturbed weather around the Bahamas.
The updated forecast primarily used a deterministic blend
(ECMWF/GFS/CMC/UKMET) for Day 3-4 with increasing weight toward
the 00Z ECENS and 06Z GEFS means by Day 6-7 given some of the
model uncertainty and spread.
...Weather Highlights/Threats...
There is a continued threat for heavy rainfall over areas from the
northern half of the Plains into the Great Lakes as upper
troughing becomes established over the northern tier states. From
the weekend into early next week the primary surface focus will be
with a cold front that pushes into the northern Plains as well as
a leading warm front. As upstream energy feeds into the mean
trough aloft, the Plains to Great Lakes part of the front may
stall for a period of time by next Tue-Wed and provide an
opportunity for training convection or repeat activity. Some
convection may be strong to severe during the period so check
Storm Prediction Center outlooks as the details become more clear
in future days.
The area encompassing the central Gulf Coast through the Southeast
and southern Appalachians/southern Mid-Atlantic will also see the
potential for areas of heavy rainfall with a combination of ample
moisture, one or more upper level impulses, and the wavy surface
front settling over the Southeast. Diurnal convection over the
Florida Peninsula may produce locally significant totals as well.
While some of the specifics of flow aloft by the middle of next
week have yet to be resolved, leading up to that point confidence
is increasing for a significant cooling trend over the northern
Plains where some locations may see multiple days with highs of
10-15F or so below normal. On the other hand areas from southern
Oregon/northern-central California into the southern High Plains
should see above normal readings for most of the period with
pockets of plus 10-15F anomalies and possibly a few daily records.
Cooler air may reach the southern Rockies/High Plains by next
Wed. Near to below normal temperatures over the eastern half of
the country during the weekend should trend toward near or
slightly above normal readings by next Tue-Wed.
Rausch/Taylor
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