Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 201025

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
425 AM MDT Thu Apr 20 2017

.SHORT TERM...(Today through Friday)
Issued at 425 AM MDT Thu Apr 20 2017

Southwest flow will increase ahead of a negatively tilted trough
moving into the Pacific Northwest and digging southeast through
the northern Great Basin. Mid and high level moisture will
increase as well with dry air at the low levels. The driest air
looks to remain across southeast Utah and southwest Colorado with
increasing mid and high level cloud cover elsewhere. The
combination of low relative humidity and gusty winds support
critical fire weather thresholds being reached in southwest
Colorado and the Paradox Valley, where fuels are critical.
Therefore, a Red Flag Warning for Colorado Fire Zones 207 and 290
remain on track for this afternoon through early evening. There
was some concern about portions of west-central Colorado,
including the Grand Valley and Montrose-Delta areas seeing fire
weather conditions. However, the increase in cloud cover along
with cooler temperatures will keep the relative humidity values
above critical thresholds.

In addition to the Fire Weather concerns, steep lapse rates of 8
to 9 degrees C/km and moderate CAPE values of 500 to 1000 J/kg
support the development of thunderstorms across the area. The dry
low levels would indicate storms initially to be high-based with
outflow winds being the primary concern before the low levels
saturate this evening after sunset. Our CWA remains in the left-
exit region of an upper level jet and the negatively tilted trough
would also support the possibility of stronger storms with small
hail also possible.

The cold front is projected to move through the region late this
evening into Friday morning. Strong cold air advection will take
place behind this cold front with H7 temps dropping to -7C to -9C
across northwest Colorado. Wind gusts to 40 mph are possible as
well ahead and with the passage of this front, but look to stay
below wind advisory criteria. Moderate to heavy precipitation
will be possible this evening and overnight as the low levels
saturate and relative humidities recover. These temperatures also
support rain or a rain/snow mix changing over to snow in the
higher elevations with snow levels lowering to mountain bases by
Friday morning. An elongated area of channeled vorticity will
stretch over northwest Colorado along with a stronger upper level
jet streak of around 110 kts on the backside of this system.
Northwest Colorado will once again be in the left- exit region of
this upper level jet and areas to the northeast of this jet and
channelized vorticity will be favored for accumulating snowfall
and the added element of convection with thunderstorm potential.
This dynamic forcing remains in place through Friday afternoon
with temperatures cold enough to support snowfall in the higher
elevations. Therefore, decided to issue a Winter Weather Advisory
for the Park and Gore Range (COZ004), the Flat Tops (COZ013) and
the Elks (COZ010) including Vail Pass for elevations above 9000
feet. Even though snow levels will lower to mountain bases and
even some higher northern valleys getting in on the action, the
greatest potential for impacts and significant snow accumulation
will be above 9000 feet, where 4 to 8 inches of snow are likely.
Locally higher amounts are possible as well due to convection.
Lesser amounts and mainly wet roads are expected below the 9000
foot level. This Winter Weather Advisory is in effect from 6 pm
this evening through 6 pm Friday. Drier and more stable air will
be moving in on the backside of this system with most
precipitation coming to an end or lessening in intensity by Friday

.LONG TERM...(Friday night through Wednesday)
Issued at 425 AM MDT Thu Apr 20 2017

A ridge of high pressure will move in for the weekend with
temperatures recovering towards above normal levels. Southwest
winds will increase on Sunday ahead of another disturbance that
will skirt the north Sunday evening through Monday morning. These
stronger winds on Sunday will also result in sufficient mixing
with lower valleys approaching 80 degrees once again. Fire Weather
concerns are possible on Sunday, as the atmosphere will be dry.

This all changes from mid to late week however as strong ridging
over the oceans looks to favor a large and cool trough digging
across the core of NOAM. An active and unsettled pattern will take
shape beginning Tuesday with this pattern still favoring a robust
Spring storm in the middle to late part of next week. This means
more moisture and precipitation in store with cooler temperatures.
Stay tuned.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 425 AM MDT Thu Apr 20 2017

VFR conditions will prevail through 18Z today. Showers and
thunderstorms will develop after 18Z with breezy southwest winds
increasing as well. Showers will become more numerous late this
afternoon into the evening as the cold front approaches. Rain or a
rain/snow mix will changeover to all snow in the mountains tonight,
favoring the northern and central Colorado mountains for best
potential of moderate to heavy precipitation through Friday
morning. CIGS will be higher initially this afternoon and will
lower this evening as the low levels saturate. Expect lower CIGS
and VSBY in showers and frequent mountain obscurations after 00Z
Friday through 12Z Friday.


Issued at 331 AM MDT Thu Apr 20 2017

Southwest winds will increase this afternoon out ahead of a
strong cold front and Pacific storm system, set to move through
the region late this evening into Friday morning. The combination
of low relative humidities and gusty winds will meet critical fire
weather thresholds in Colorado Fire Zones 207 (southwest Colorado)
and 290 (Paradox Valley), where fuels are critical. This drier
air will also filter into west-central Colorado but slightly
cooler temperatures and cloud cover will keep relative humidities
above critical thresholds. Behind the front, surface winds will
die down and humidities will rise back above critical thresholds.


CO...Red Flag Warning from 1 PM this afternoon to 8 PM MDT this
     evening for COZ207-290.

     Winter Weather Advisory from 6 PM this evening to 6 PM MDT
     Friday for COZ004-010-013.



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