Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1103 AM EDT Sun Jul 11 2021
Valid 12Z Wed Jul 14 2021 - 12Z Sun Jul 18 2021
...Western Heat Persists This Week and Spreads into the Northern
Plains...
...Midwest Heavy Rainfall Threat to Redevelop Midweek...
...Weather Highlights/Hazards...
Yet another significant heat wave now underway over the West under
upper ridging will continue from the Great Basin to CA and the
Southwest with temperatures 10-15F above normal into midweek. This
will produce some daily temperature records. Guidance consensus is
leading to gradually lowering heights with a developing modest
northwest US trough, with the highest heights shifting south into
southern CA/AZ. There is potential for rebuilding heights over the
Great Basin to the central Rockies and northern Plains as a closed
low persists off the west coast of Canada next weekend. Above
normal temperatures are expected across much of the northern Great
Basin and northern Rockies/High Plains next Fri 16 July- Sun 18
July, with cluster of high temperatures reaching 10-15 degrees
above normal next weekend.
Lingering upper ridging over the western Atlantic and into the
Southeast may also support above normal temperatures this week
from the Mid-Atlantic to the lower Great Lakes and into the
Northeast. This includes the Washington, DC to Baltimore,
Philadelphia, and New York City corridor. Over next weekend, the
upper trough drifts east from the Lakes toward the northeast,
driving a cold front across the region. This increases
shower/storm chances with the front, followed by post-frontal
cooling in the lower Lakes and then next Sunday 18 July into the
adjacent northern mid Atlantic/New England.
The forecast does indicate potential for temperature moderating
showers/thunderstorms over Arizona/Southern Rockies this week with
pockets of enhanced 700 mb relative humidity in place to support
diurnal activity with instability peaking each afternoon/evening.
This may lead to local downpour and terrain enhancing runoff
issues, extending from short range activity.
With a wave front in the Midwest to the central Plains, additional
rounds of heavy showers and storms are forecast near the
meandering front, instability and upwards of 1.75" precipitable
water values. The models have renewed rainfall maxima gradually
shifting from the Upper Midwest by midweek to adjacent portions of
the Mid-MS/OH Valleys late week with slow frontal progression. The
rain threat area drifts south next weekend down the plains, MS
Valley and from the OH Valley into the TN Valley, central-southern
Appalachians, and mid Atlantic.
...Guidance/Predictability Evaluation...
Guidance forecast spread and uncertainty remains below normal next
week with a main exception of shortwaves progressing through a
mean Midwest trough and whether a closed low develops within the
500 mb trough next weekend near the Lakes/adjacent Canada. The WPC
medium range product suite was primarily derived from a blend of
best clustered guidance from the 06 UTC GFS and 00 UTC
ECMWF/UKMET/Canadian Days 3-5 (Wed-Fri) along with the 13 UTC
National Blend of Models. The compatible 00 UTC GEFS/ECMWF
ensemble means were then mainly used day 6/7 (next weekend) versus
the models amid growing run to run variance. The WPC composites
maintain act to mitigate these slowly growing smaller scale
guidance differences consistent with predictability.
Petersen/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