Scientific Forecaster Discussion
fxus65 kriw 172300
Area forecast discussion
National Weather Service Riverton Wyoming
400 PM MST sun Nov 17 2019
Short term...this afternoon through Monday night
A Flat Ridge has built back over the epac, with a high center off
the central California coast. A 120 to 140kt jet will be moving through this
ridge, with a stream of Pacific moisture over the pacnw and northern
rockies this morning. The ridge will amplify through the day,
bringing the jet over the state by this afternoon. Minor shortwaves
embedded in the cloud shield over the northern rockies, will make
their way southward through the day today. This will bring snow
showers across northern portions, spreading southward into Natrona
County this afternoon and evening. Snow accumulations will be very
light, with a few inches on the west/northwest slopes above 8000
feet of the Bighorn and Absaroka Mountains. Otherwise, gusty
winds will occur over the mountains and the wind corridor between
Rock Springs and Casper. Northwest flow will continue over the region
Monday, as the ridge shifts eastward. This will be in response to
a developing storm system that will be making its way to the pacnw
Monday. Dry and mild conditions will occur across the forecast
area Monday, with gusty winds continuing over southern Wyoming.
Long term...Tuesday through Sunday
The flow aloft will transition from northwest to west on Tuesday in
response to an upstream trough moving onshore along the West Coast.
700mb temperatures Tuesday will still be at or above 0c which will
translate to high temperatures 10f to 15f above normal. Only
limiting factor would be increasing high clouds which could hinder
solar heating, especially where standing wave clouds form downstream
of the mountain ranges. Gusty southwest surface wind across central
Wyoming could boost readings in Casper above 60f again. A cold front
will reach the far west late in the day Tuesday, but not early
enough to significantly impact high temperatures. Late day rain/snow
showers Tuesday should mainly be confined to the northwest mountains.
The trough energy splits with a northern stream shortwave tracking
along our northern border Tuesday night. The bulk of the energy
within the trough will dive to the Desert Southwest. The shortwave
and the overall pattern will combine to drag a cold front south
through the forecast Tuesday evening and overnight. Forecast three-
hour pressure rises around +6mb should generate some gusty Post-
frontal wind across northern portions of both the Bighorn Basin and
Johnson County. Wind speeds in these areas were adjusted upward
Tuesday night and Wednesday to account for the expected higher wind.
Substantially colder air will move south in the wake of the cold
front. 700mb temperatures Wednesday will be around -11c, a far cry
from those seen Tuesday. The front will push through our southwest
zones Wednesday morning in response to lowering pressure to our
south. The scenario setting up is one where the upper low over the
Desert Southwest will spread moisture north over the top of this
colder air. Central and southwest Wyoming look like the best areas
for accumulating snow during the day Wednesday on into Wednesday
evening. Nudged quantitative precipitation forecast upward toward wpc guidance during this period
with 0.10 to 0.20 inches looking more likely. Upslope component,
favorable mid-level temperatures, and possible right-entrance region
jet dynamics could all combine to generate a period of moderate snow
Wednesday in central Wyoming. Temperatures may struggle to reach
those currently forecast if the precipitation begins early enough
and dynamic cooling occurs. Regardless, precipitation will be all
snow with lower elevation accumulations of a up to a few inches
looking quite possible. Surface winds 15 to 25 mph will be common
with slightly stronger northeast winds across the I-80 corridor.
Deformation axis between the northern and southern energy streams
will drift south across the state during the day Thursday. This will
bring improving and drying conditions from north-to-south Thursday.
Assuming fresh snow cover and the clearing sky, temperatures
Thursday night should be colder than model guidance. Have adjusted
temperatures downward accordingly. The Desert Southwest low will
eject across the southern rockies Friday as a ridge builds in across
the western U.S. Friday and Saturday look dry and seasonal with this
upstream ridge in place. The next chance for precipitation comes
Sunday and Sunday night across mainly the northwest quarter of the
Aviation...for the 00z tafs through 00z Tuesday
Cloud cover has become more extensive across the forecast area
Sunday afternoon as a shortwave approaches in northwest flow aloft.
Intermittent bkn040-050agl ceilings persist across the far west,
including at kjac, kbpi, and kpna. Showers are becoming more
widespread across the northwest quarter of the state as the upstream
shortwave is set to cross the state late this afternoon and evening.
Mountain top obscurations will be frequent across the northwest and
northern mountains and prevail through the evening. Best chance for
showers would be at kjac, kcod, and kwrl through about 04z/Mon.
Models still hinting at some late evening vcsh at kcpr and have
continued vcsh there. VFR conditions will be the general rule
through Monday. The gusty southwest wind at krks will decrease
slightly after sunset, but will persist through Monday. Kcpr and
kcod gusty winds to decrease early Monday morning.
Another quick-moving weather disturbance will track across Wyoming
late today and tonight. This system will produce snow showers across
the northwest and northern mountains as it passes through. Scattered
light rain showers are also possible in the Bighorn Basin and across
Johnson and Natrona counties Sunday evening. Most zones will see a
mostly clear sky by sunrise Monday as this system exits the region.
Monday will be dry and warm with breezy conditions across the High
Desert district into Natrona County. The next weather system and
associated cold front will reach northwest Wyoming late Tuesday.
Rain and snow showers will mainly be confined to the northwest
mountains and far north late Tuesday and overnight. Tuesday
temperatures will remain unseasonably warm across the east and
southwest ahead of the cold front. Gusty southwest wind from
Sweetwater County to Natrona County could boost temperatures a few
more degrees Tuesday afternoon. Across the far west, temperatures
will begin to cool down late in the day Tuesday in the wake of the
cold front. Much colder air will filter into the state Tuesday
night and Wednesday with temperatures falling to below normal.
Moisture to the south of Wyoming will ride over the top of this
colder air and bring light snow to many areas east of the
Continental Divide Wednesday and Wednesday evening. The end of the
week looks drier but temperatures will remain on the cool side.