Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 250905
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
305 AM MDT Tue Apr 25 2017

.UPDATE...
Issued at 305 AM MDT Tue Apr 25 2017

Snow depth sensor at Skyway Point on the Grand Mesa registered 4
inches of new snow in the past several hours which seemed
reasonable given the broad band of showers over the area late last
evening and earlier this morning. Moist northwest flow and
instability should bring additional snowfall to the area during
the day. Consequently, added CO Zone 9 to the Winter Weather
Advisories issued earlier. A more complete discussion will
follow in the next hour or so.

UPDATE Issued at 950 PM MDT Mon Apr 24 2017

Quick update to the grids to adjust pops through the remainder of
the night into the morning hours on Tuesday. Precipitation
becoming more efficient across the northern zones with activity
expected to sag south into the central and southern mountains
later tonight. Valleys will continue to see isolated to scattered
rain showers, although some snow possible across the Northwest
Plateau, including, Craig, Meeker and Hayden. Temperatures already
down into the mid 30s so some mixing likely occurring already.
Webcams are sparse in this area, so relying on spotter reports and
sfc obs. Forecast snowfall amounts in the 4-8 range still look
reasonable above 9k with some overnight impacts to transportation
likely along the higher passes. Will take another look at the
Grand Mesa for potential addition to the highlight reel, otherwise
current highlights are in good shape.

&&

.SHORT TERM...(Tonight through Tuesday night)
Issued at 310 PM MDT Mon Apr 24 2017

Clouds and moisture are increasing across the forecast area under
a fast west to east flow driven by a 140 kt jet streak over the
Pacific Northwest that is translating to 100 kts over our area.
Mixing is occurring despite extensive cloud cover with breezy
conditions at many locations, yet due to increased moisture, the
relative humidity values are staying above critical thresholds so
fire weather concerns are minimized today. Expect showers to favor
the northeast Utah and northwest Colorado this afternoon with
isolated thunderstorms possible but CAPE appears fairly low around
200 J/kg. Precipitable Water (PW) values increase towards 0.4 to
0.6 inches across the area with specific humidity values in the 4
to 5 g/kg range, so plenty of moisture to work with.

A weak low circulation develops over Wyoming this evening keeping
the northern and central mountains favored for snowfall, while
the main low over Oregon opens up and slides across the Great
Basin and our area by Tuesday afternoon. This will shift the focus
towards the central and southern mountains for snowfall as the
flow shifts to northwest behind the cold front and trough passage.
Rain will primarily occur at the lower elevations Tuesday
afternoon with better potential for thunderstorms as CAPE
increases towards 400 to 800 J/kg and lapse rates steepen towards
8 degrees C/km on average. Showers decrease in coverage by late
Tuesday evening into Tuesday night as the system exits and drier
air moves in on the backside with upper level forcing waning.
Expect the best periods of moderate to heavy snowfall to occur
with each frontal passage with the two aforementioned lows: the
first tonight into Tuesday morning over the northern and central
mountains, and the second Tuesday afternoon/evening over the
central and southern mountains. H7 temps only cool down to as low
as -4C to -6C with this system, with a slight moderation Tuesday
afternoon before cooling again, so expect impacts and best
snowfall accumulations associated with this system to remain
mainly above 9000 feet. Winter Weather Advisories remain on track
for 4 to 8 inches of snow mainly above 9000 feet for the western
Colorado mountains through Tuesday evening.

.LONG TERM...(Wednesday through Monday)
Issued at 310 PM MDT Mon Apr 24 2017

After a break Wednesday morning while in between weather systems
in this unsettled pattern, moisture and showers increase once
again by late Wednesday afternoon into Wednesday evening as the
next system approaches. The upper level jet stays oriented from NW
to SE across the Pacific Northwest towards the Four Corners during
this period, keeping our CWA in the left exit region of the upper
level jet. The strongest jet streak is over the Pacific Northwest
of 170 kts which translates down towards 110 kts over the Four
Corners. The cold front associated with this system moves across
early Thursday morning with northwest flow lingering through
Thursday afternoon and evening, favoring the northern and central
Colorado mountains. Another round of decent snowfall is possible
Wednesday evening through Thursday afternoon, so will need to keep
an eye on the details as we near closer to the event. One concern
is that a ridge of high pressure will amplify over the Pacific
Ocean, pushing the upper level jet further northeastward, allowing
drier air to traverse from the northwest and effectively cut off
the moisture supply and put an end to most precipitation by
Thursday afternoon.

As this Pacific ridge amplifies and pushes further east, this
also amplifies the troughiness over the Intermountain West.
Previous forecasts were indicating the potential for a significant
storm system to develop by late in the week and impact the area
through this weekend. The latest 12Z guidance is now showing this
low developing further east than previous runs with the low
developing over the northern Utah- Colorado border Friday morning
and tracking southward towards the Four Corners by Friday evening.
This brings increasing chances of widespread precipitation across
the area and colder air advecting in with H7 temps lowering
towards -10C Friday morning through Saturday morning. There is
potential for below freezing temperatures in the lower valleys so
something to keep an eye on.

The amplified Pacific ridge moves further eastward into the
western states and draws concerns for the going forecast in terms
of significant snow potential for the mountains with this system
as this also draws much drier air into the region during this
period. This further eastward track also brings the focus for best
precipitation onto the Front Range and Plains in the southwest
flow ahead of the system on Saturday. PW values remain less than
0.25 inches from Thursday afternoon through at least Sunday, so
this doesn`t bode well for significant precipitation. The ridge
helps to quickly push this system out of the region towards the
Plains by Saturday evening with an unsettled but drier northwest
flow over the area Sunday and Monday, keeping the northern
Colorado mountains favored for continued orographic snowfall.
Still kept the PoPs on the conservative side for late in the week
as even though the 12Z GFS and EC are in good agreement with this
further eastward and drier solution, run to run consistency is
lagging.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1117 PM MDT Mon Apr 24 2017

Widespread rain and snow showers will remain in the forecast
through Tuesday evening. Expect ILS cigs at all sites with areas
of IFR/MVFR cigs and vsbys. Mountains will be obscured through
Tuesday evening. Conditions improve from west to east Tuesday
night.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Winter Weather Advisory until 9 PM MDT this evening for COZ004-
     009-010-012-013-018-019.

UT...None.

&&

$$

UPDATE...NL
SHORT TERM...MDA
LONG TERM...MDA
AVIATION...JDC


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