Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
230 AM EDT Thu Jun 21 2018
Valid 12Z Sun Jun 24 2018 - 12Z Thu Jun 28 2018
...North-Central U.S. Heavy Convective Rainfall Pattern...
...Guidance and Predictability Assessment...
Models and ensembles continue to show above normal agreement with
the overall medium range flow pattern evolution, bolstering
forecast confidence. Nrn stream flow features a slowly
translational e-central Canada upper ridge bookended by amplified
w-central Canada/NW U.S. and ern Canada/NE U.S. cooling troughs. A
modestly progressive pattern underneath for the CONUS has a
tendency for embedded system amplification. Energy digging through
the Rockies to the n-central U.S. tends to deepen/close off in a
region of relatively weak upper flow beneath the Canadian ridge.
This seems the case again Sun-next Thu when ample system height
falls and instability/pooling moisture set the stage for slow
moving convective clusters that would lead to a locally excessive
rainfall threat centered from the central plains to the mid-upper
MS valley states. A mix of a few GFS/Canadian model runs over the
past few days have also dabbled with the threat of closing off a
Northeast/Ern U.S. trough with organized coastal lows as early as
Mon/Tue, but run to run variance in these models with these
features have been less than stellar. Accordingly, the WPC medium
range product suite was mainly derived from a composite blend of
reasonably clustered and consistent guidance from the latest ECMWF
and GEFS/ECMWF ensemble means. This maintains excellent WPC
continuity.
...Weather Highlights and Threats...
Lead trough/low energy continues to shear northeastward over the
Northeast then Canadian maritimes Sunday. Post-frontal high
pressure will settle a moderating and wavy trailing front far down
over the ern U.S. next week and focus some convection. Upstream, a
dominant amplifying shortwave trough digs from the NW U.S.
southeastward over the Rockies to the north-central states. This
system will produce an organized surface low along a main frontal
boundary. Convective complexes/heavy rains will focus along the
front and into the warm sector fueled by pooled moisture.
Meanwhile further upstream, a late season cooling front with much
more modest pcpn shows good promise to work earnestly from western
Canada down across the NW U.S. and n-central Great Basin/Rockies
next week as supported by amplified mid-upper level trough
amplification and passage to the north.
Schichtel
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities 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