Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
105 AM EDT Sun Jul 11 2021
Valid 12Z Wed Jul 14 2021 - 12Z Sun Jul 18 2021
...Western Heat Persists Thru Next Week and Spreads into the
Northern Plains...
...Midwest Heavy Rainfall Threat to Redevelop Midweek...
...Weather Highlights/Hazards...
Another significant heat wave now underway over the West under
upper ridging will continue from the Great Basin to CA and the
Southwest into early-mid next week with temperatures 10-15F above
normal. Several daily temperature records are likely into
Tuesday/Wednesday. 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 next 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 into 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.
The forecast does indicate potential for temperature moderating
showers/thunderstorms over Arizona/Southern Rockies next week with
pockets of enhanced 700 mb relative humidity in place to support
diurnal activity with instability peaking each afternoon/evening.
However, this could also lead to additional local downpour and
terrain enhancing runoff issues.
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...
Model and ensemble forecast spread and uncertainty remains below
normal next week with 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 18 UTC GFS/GEFS, 12 UTC
ECMWF/UKMET and ECMWF ensembles along with the 01 UTC National
Blend of Models. The composite maintains good WPC continuity and
acts to mitigate 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