Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 072048

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
148 PM MST Thu Dec 7 2017

.SHORT TERM...(Tonight through Friday night)
Issued at 148 PM MST Thu Dec 7 2017

Expect light snow to continue this evening across the northern and
central Colorado mountains, with some flurries in the adjacent
valleys in NW Colorado, as a weak shortwave drops through from the
north. Not much accumulation expected with a couple inches at
best in the higher terrain and at the higher peaks as the
atmosphere remains dry and cold. Specific humidity is around 2
g/kg which is not very high for this time of year with H7 temps in
the dendritic zone of -10C to -16C, resulting in light fluffy
snow. Conditions rapidly dry out behind this system as the ridge
of high pressure strengthens to the west and shifts a little
eastward, with dry northerly flow encompassing the area by late
Friday morning. Any lingering light snow or clouds will hang
around over the divide areas through sunrise and dissipate shortly
thereafter. Expect mostly sunny skies with below normal
temperatures on Friday. The cold, northerly flow underneath
building high pressure will allow mid-level inversions to
strengthen according to model soundings, so even though temps will
slowly be warming aloft, temps will be slower to warm in the lower
valleys underneath these strong inversions. Therefore, lowered
temps across the board and trended more towards non-bias corrected
guidance once again as it appeared more reasonable given the
synoptic situation. Overnight lows both tonight and Friday night
will be at their coldest as this system exits and skies clear with
efficient radiational cooling taking place. Trended below guidance
in some of the lower valleys that have a tough time breaking
inversions like Gunnison.

.LONG TERM...(Saturday through Thursday)
Issued at 148 PM MST Thu Dec 7 2017

Same story, different day...amplified ridge of high pressure
persists through the end of the long term period and beyond
through at least mid-December, showing very little change, meaning
dry conditions and trends towards above normal temperatures.
There will be some subtle shifts as weak shortwaves rotate all the
way around the periphery of this high pressure ridge and weaken
as they come around to our neck of the woods, resulting in
increasing high clouds and nothing more. Thinking the models are
wanting to warm temperatures up too quickly though this weekend
into next week as the mid-level inversions remain and slowly erode
heading into next week. Therefore, trended towards cooler non-bias
corrected guidance for both daytime highs and overnight lows,
slowing this warming trend. Temperatures should warm to above
normal levels by the weekend into next week, but slower than
models predict. There are hints at a weak shortwave brushing by
the northern divide Thursday with some light snow over the
mountains. However, this ridge remains fairly strong with
continued dry air so not expecting much as models may trend drier
as they have been lately. Not biting off more than we can chew at
this point as hopes for significant snow in the near future remain
very slim and hard to predict this far out when and if this
blocking pattern will break down any time soon.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1041 AM MST Thu Dec 7 2017

Primary forecast challenge this afternoon is whether or not to
include -SN at ASE. Have opted to add some light snow around 17z
given current radar trends, latest report of -SN at EGE, and
webcam imagery from the ski resort showing snow currently falling
higher up in the resort. Infrared satellite imagery reveals the
coldest cloud tops currently from EGE and ASE eastward to the
divide and it is most likely that if -SN is to affect these
terminals, it will do so in the next few hours. CIG heights are
likely around their lowest for the event, and will generally
begin to rise after 00Z this evening. Clouds will clear overnight
leading to a mostly sunny morning on Friday.




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