Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 11/12/20
Area Forecast Discussion
National Weather Service Grand Junction CO
434 PM MST Wed Nov 11 2020
.SHORT TERM...(Tonight through Thursday night)
Issued at 230 PM MST Wed Nov 11 2020
One minor ripple in the broader upper level trough over the region
is crossing northwest CO at this time. coincident weak elevated
instability will peak between now and late afternoon, and as the
HRRR and NAMNest models have pegged, convective bands have
developed across far NW CO. Expect these to continue through early
evening, impacting the mountains of NW CO with areas of less than
2 inches of snow accumulation.
The next shortwave trough over eastern ID right now will combine
with a strengthening upper level jet coming west to east across UT
tonight to provide enhanced lift (in addition to orographics)
across the same areas of northwest CO, but will also allow for
scattered snow showers farther south such as over the Elks and
from Vail Pass to Monarch Pass. Available moisture is somewhat
limited, but high snow-to-liquid ratios should yield a total of 3
to 6 inches of snow north of Rabbit Ears Pass and 1 to 3 inches
over the Flat Tops and over the Gore and Elk Ranges. Gusty winds
to 45 mph over those higher elevations will cause blowing and
drifting snow as well. Otherwise, it will be a mostly clear to
partly cloudy and cold night for the rest of the area.
This system will exit Thursday morning and the snow shower
activity along the Divide will diminish. Northwest flow will
transition to zonal during the day into Thursday night.
Temperatures will remain about 10 degrees below normal.
.LONG TERM...(Friday through Wednesday)
Issued at 230 PM MST Wed Nov 11 2020
A stronger and wetter trough will plow into the Pac NW Friday and
bring stronger forcing over northeast UT and much of western CO
centered on late Friday night into early Saturday morning.
Westerly upslope flow will be enhanced by this dynamic lift and
moisture content will be greater. Snow chances will increase over
northeast UT and northwest CO Friday evening and expect the
highest snowfall rates between 09-15Z with 6-hourly rates of 3-5
inches...highest over the Gore, Park, Flat Top, and Elk Ranges.
This event may require an advisory for those ranges considering
8-10 inch totals and considerable blowing snow. With the
strongest forcing and best moisture remaining north, the San Juan
mountains will be lucky to receive 1 to 4 inches of snow. Lower
elevations of west-central and northwest CO and northeast UT will
likely see up to an inch, but the I-70 corridor of the Grand
Valley looks to stay mainly dry. Snowfall will diminish Saturday
afternoon and night in the wake of the trough axis passage and the
northwest upslope flow dries out. However, light snow showers
will likely persist for the northern ranges of the Divide through
Upper level heights rise early next week as a ridge aloft develops
over our region and temperatures will rise above normal by
Tuesday...with these same dry and above normal temperatures
.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 427 PM MST Wed Nov 11 2020
A disturbance clipping northeast Utah and northwest Colorado will
continue to produce scattered and intermittent light snow
overnight, particularly at KHDN and to a lesser extent KVEL, KEGE
and KASE. During periods of snow, expect reduced visibility and
ceilings that approach ILS breakpoints. Most snow showers will end
Thursday morning, with a gradually clearing sky to follow.
Otherwise, VFR conditions expected elsewhere for the next 24
hours. Winds will generally be light and terrain driven.
AREA FORECAST DISCUSSION
National Weather Service Morristown TN
905 PM EST Wed Nov 11 2020
Cold front is moving through the area this evening with winds
shifting to the NNW/N behind the front at obs sites. Rain
continues to shift east with highest PoPs early tonight across the
eastern half of the forecast area. This rain will continue
shifting east overnight with PoPs decreasing late tonight and
ending for most areas by daybreak Thursday. Some drizzle may
linger early Thursday with some light rain showers across
the higher elevations due to orographic lift, but precip will be
isolated and decrease quickly through the day.
With the low FFG and continued moderate rain, will continue the
FFA at this time. Most of the moderate precip will be exiting the
area in the next hour, and if this trend continue, may cancel the
watch prior to the 12z expiration.
00Z TAF DISCUSSION.
Poor flight conditions forecast overnight as a cold front with
widespread rain slowly moves east across the area. Steady rain is
coming to an end at CHA but will continue for the next 6-12 hours
at TYS and TRI. Even after rain has ended at sites, cigs will
remain low with some -dz through Thursday morning. General
improvement in conditions is expected near the end of the TAF
period as drier air arrives.
/ISSUED 319 PM EST Wed Nov 11 2020/
SHORT TERM (Tonight and Tomorrow)...
Showers, with a few rumbles of thunder, will decrease west to east
tonight with lingering chances tomorrow in the Tennessee/North
Carolina mountains. Locally heavy downpours and isolated flooding in
low-lying and poor drainage area is still possible. Temperatures
will be seasonally mild but closer to seasonal norms.
Current Setup: Currently, the long-discussed cold frontal boundary
is located along western areas of the County Warning Area (CWA).
Aloft, mid/upper-level troughing is dominating in the western two-
thirds of the CONUS with tropical cyclone Eta located ~50 miles west
of the western Florida coast.
Throughout the evening, eastward progression of the frontal boundary
will continue with the CWA being located at the right-entrance
region of a 60-80kt jet streak. Low-level convergence associated
with the frontal boundary and upper-level divergence enhanced via
direct ageostrophic circulation in the upper jet will both
provide continued forcing for convection. Deep moisture remains in
place ahead of the frontal boundary with post-frontal low-level
moisture evidenced by 925mb-850mb saturation. By this evening, the
front will be located along the eastern periphery of the area.
Given the deep moisture and marginal dynamics, PoPs will remain
high into the evening. RAP indications of pre-frontal surface-
based instability above 500 J/kg will carry slight chance of
thunder into the early evening. Pre-frontally low LCL heights with
freezing levels above 14,000 feet, combined with the
aforementioned considerations, suggests that notable rainfall
rates are possible in heavier showers and thunderstorms, which
has already been seen along the Cumberland Plateau. Also, SW flow
has allowed for some training of cells with eastward movement
increasing. Given these characteristics and latest high-res
indications, localized flooding continues to be possible with an
additional 1 to 2 inches possible in eastern portions of the area
with locally higher amounts possible in the Tennessee mountains
and into Washington County, VA. Based on lower flash flood
guidance values, a Flood Watch is in effect in the mountains of
Blount County up to northeast Tennessee and Washington County, VA
until tomorrow morning. This is based on expected rainfall of 2 to
3 inches or more, ground saturation, and post-frontal upslope
Heading into Thursday, much of the troughing and jet dynamics remain
to the north. Deep moisture will have pushed off to the east with
lingering low-level moisture and NW flow keeping slight
chance/chance PoPs in the eastern high elevation areas before the
moisture finally moves off to the east at the end of the period.
Otherwise, seasonal temperatures prevail, given continued cloud
cover and mid/upper heights close to normal.
LONG TERM (Thursday Night through Wednesday)...
1. Dry weather returns Friday and Saturday with plenty of sunshine,
and temps will still average 4 to 8 degrees above normal.
2. Showers will move through the region Sunday into Sunday night,
but rainfall amounts will be light.
3. Dry conditions return early next week, but temps will finally
cool to near normal.
Thursday Night through Saturday Night...
Drier air will work into the region Thurs night as surface high
pressure builds in from the N and W. The cold front will only slowly
shift E through the Carolinas since the mid/upper flow remains
somewhat WSW ahead of another shortwave digging into the desert SW.
At the same time, the remnants of Eta will also move along the
boundary near the GA and CA shoreline further slowing the front.
This will cause some moisture to linger over the far eastern
mountains, so clouds and a few stray showers will in that area Thurs
night. The surface high will win out Fri with mostly sunny skies
developing areawide. Since the upper trough stays well to our N
moving across the Great Lakes and the flow over our region remains
WSW, highs will remain mild in the mid/upper 60`s. The
aforementioned desert SW shortwave will phase with a stronger
shortwave crossing the Rockies into the Plains by Sat which will
rebuild downstream ridging across the TN Valley and SE CONUS, with
the surface high shifting to the E coast. This will set up southerly
flow, with the resultant WAA boosting highs into the upper 60`s to
near 70 for Sat making for a beautiful start to the weekend. Clouds
will begin to increase ahead of the system Sat night.
Sunday and Sunday Night...
The mid/upper shortwave will move from the Plains across the Midwest
and Great Lakes Sun and Sun night. Its axis will become negatively
tilted resulting in a deepening surface low moving from the Great
Lakes into Ontario, with the associated strong cold front crossing
our CWA Sun. Deterministic and ensemble guidance have come into good
overall agreement on a afternoon and evening fropa. Lift from a 150
kt upper jet over the OH Valley will combine with warm, moist
advection ahead of the front to produce a band of showers, but the
surface high holding on for so long near the E coast and relatively
fast moving nature of this system will limit the quality of moisture
return. This will result in the showers being tied closely to the
boundary limiting QPF to a trace to a few hundredths of an inch.
Kept PoPs in the chance range Sun. Drier air will quickly pour in
behind the front Sun night, so most areas will dry out except for a
few lingering showers from SW VA down the eastern mountains. Temps
will remain warm ahead of this front with upper 60`s/low 70`s Sun
before falling sharply into the low 40`s Sun night in the post fropa
Monday through Wednesday...
A fairly deep but progressive mid/upper trough will dig into the
eastern CONUS behind the system Mon through Tues with strong surface
high pressure centered near the MS Valley. This will place our
region in dry, cool NW flow with highs near normal in the mid 50`s
and overnight lows in the 30`s. Slight moderation will occur by Wed
as the upper trough begins to lift out and height rises take place.
.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN 58 70 49 68 44 / 50 10 0 0 0
Knoxville McGhee Tyson Airport, TN 57 66 49 65 41 / 80 10 0 0 0
Oak Ridge, TN 55 66 46 66 41 / 70 10 0 0 0
Tri Cities Airport, TN 57 64 50 64 37 / 90 30 10 0 0
TN...Flood Watch until 7 AM EST Thursday for Blount Smoky Mountains-
Cocke Smoky Mountains-Johnson-Northwest Carter-Sevier Smoky
Mountains-Southeast Carter-Southeast Greene-Sullivan-Unicoi-
VA...Flood Watch until 7 AM EST Thursday for Washington.