Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Wed Jun 06 2018
Valid 12Z Sat Jun 09 2018 - 12Z Wed Jun 13 2018
...Pattern overview...
A northern stream omega block signature is noted in the guidance
with the key player being a powerful upper trough approaching the
Pacific Northwest early in the weekend. Characterized by 500-mb
height anomalies around 2 sigma, an embedded closed low/strong
band of height falls will slide eastward reaching the Northern
Rockies by 11/1200Z. Ultimately this should suppress a mean upper
ridge which will sprawled across the Desert Southwest eastward to
the Southern Plains. Gradually this positive height anomaly will
rebound by early next week with real estate generally covering the
southwestern quarter of the country. To the north, the mentioned
upper trough pushing through the Northern Rockies on Monday is
expected to slow considerably and may even close off at some point
near the Dakotas/south-central Canada border. All the while, this
omega block set up will shift the pattern downstream with broad
upper troughing across the northeastern U.S. gradually sliding
offshore by the middle of next week.
...Model guidance/predictability evaluation/preferences...
Initially focusing on the upper trough barreling through the
Pacific Northwest, multi-day ensemble spaghetti plots have shown a
faster trend. On Monday morning, the mean trough axis is roughly
located over central Montana while three model cycles ago it was
back toward the Pacific Northwest. Previous days offered random
operational solutions which appeared more outlandish. However,
through Day 5/June 11, the guidance has been fairly consistent
which bolsters confidence across this region. The attendant height
falls help erode the northern extent of the mean ridge over the
southwestern to south-central U.S. with the 12Z CMC and 00Z/12Z
UKMET solutions holding on to the ridge a bit longer. As the
heights relax once the parent upper low moves toward the
Saskatchewan/Manitoba border, a return of the semi-permanent ridge
is likely during the Day 5-7, June 11-13, timeframe. It should
generally re-assume a similar position with a 588-dm height
contour stretching from the Desert Southwest toward the
Southern/Central Plains. Considering the strong feature
accelerating through the Northern Rockies on Monday, a great deal
of uncertainty looms thereafter as solutions waver about multiple
scenarios. The 00Z GFS has made a shift toward a more progressive
solution after most guidance favor closing off the upper low
somewhere across south-central Canada where a quasi-stationary
motion is attained. Ensemble spaghetti plots vary considerably
which promotes more emphasis on the global ensemble means. Farther
downstream, the eastern extent of the omega block will drift
eastward with the level of amplification up for debate. Some
fairly amplified solutions are in the cards although trusting any
one operational model would not be wise here. As an example, at
13/0000Z, the 00Z GFS keeps height falls restricted to New England
while the preceding run shifts down to the lower mid-Atlantic
region. However, at the very least, the ultimate synoptic
configuration appears reasonable across the board.
Regarding preferences, through Day 4/Sunday, was able to utilize a
multi-operational model blend led by the 18Z/12Z GFS and 12Z ECMWF
with minor contributions from the 12Z CMC/UKMET. Thereafter,
gradually incorporated heavier weighting of the ensemble means
with only a 20 percent influence from the 18Z/12Z GFS and 12Z
ECMWF given significant uncertainties within the northern stream
omega block. One region which remains stable is the
re-establishment of the upper ridge across the southwestern U.S.
with ensemble means in good agreement there.
...Weather highlights and hazards...
While much of the country can expect above average temperatures
during the period, there will be a few exceptions to this rule. In
particular, expect numbers around 5 to 15 degrees below
climatology this weekend from northern California and the Pacific
Northwest eastward into the Central Great Basin/Upper
Intermountain West. Many high temperatures will struggle to get
out of the 50s with lows in the 30s across the Central Great
Basin. Further east, deep troughing across the northeastern U.S.
will favor slightly cooler temperatures, with similar impacts
across Florida except driven more by extensive cloud cover and
rainfall. Otherwise, the rest of the country will remain rather
warm, especially over the middle of the country in advance of the
barreling upper low. Primarily an issue this weekend, departures
from climatology will range from 10 to 15 degrees across the
Northern/Central Rockies to the adjacent Great Plains.
Multiple mesoscale convective complexes are likely to develop
north of the prevailing ridge in the form of "ridge rollers".
Predictability of any such episode of course remains low but
enhanced rainfall is expected anywhere from the Middle Mississippi
Valley downstream to the Ohio Valley. There will also be enhanced
rainfall across the Lower Great Lakes into the northeastern U.S.
for much of the weekend, particularly along the west-east oriented
frontal zone. The focus for convection will shift southward into
early next week as the frontal zone moves south of 40N latitude.
Farther west, enhanced onshore flow and seasonably strong dynamics
will support widespread rainfall from the Pacific Northwest into
the Upper Intermountain West. Local orographics should help focus
higher amounts where upslope flow prevails. As the frontal zone
shifts toward the middle of the country, expect renewed convection
across the north-central states into next week.
Rubin-Oster
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be 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