Area Forecast Discussion
Issued by NWS Grand Junction, CO

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
FXUS65 KGJT 171058

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
458 AM MDT Sat Mar 17 2018

.SHORT TERM...(Today through Sunday)
Issued at 458 AM MDT Sat Mar 17 2018

Southwest flow is increasing across the region this morning as
another stronger Pacific storm system moves into the Great Basin.
Afternoon temperatures today will be milder than yesterday with
some cumulus clouds developing mainly over the higher terrain in
this warm advective flow. Specific humidities increase to between
3 and 3.5 g/kg across eastern Utah with some orographic snow
showers beginning over the eastern Uinta mountains this afternoon.
Conditions will be breezy with H7 winds between 20 and 30 kts. Not
much precipitation expected aside from the eastern Uintas this
afternoon, but steep lapse rates of 7 to 9 degrees C/km could lend
themselves to cumulus buildups over the high terrain and perhaps a
virga shower or two as the low levels remain dry. Moisture
increases significantly with top down saturation occurring this
evening across northeast Utah and spreading into southeast Utah
late tonight into early Sunday morning. This will allow showers to
begin across southeast Utah with snow beginning in the La Sals and
Abajos and rain in the lower valleys, before mixing in with some
snow and changing over by daybreak on Sunday morning. This
moisture and precipitation is expected to move into western
Colorado by Sunday morning and spread northeastward as the trough
axis moves through.

This system is bringing slightly colder air than the previous
system, but the difference is that temperatures prefrontal are not
as warm. The cold air also comes in late this evening across
eastern Utah and advects into western Colorado by Sunday morning
with H7 temps lowering to about -7C by Sunday morning, heading
into the -8C to -10C range by Sunday afternoon and bottoming out
between -10C and -12C by Sunday evening into Monday morning.
Moisture levels also stay up a bit post frontal with specific
humidity values in the 2 to 3 g/kg range, which is still decent.
Lapse rates remain steep through Sunday afternoon in the 7 to 9
degrees C/km range, with some low CAPE in the realm of 200 to 300
J/kg, which is actually good enough for this area and this time of
year to produce convection and thunderstorms. This convective
element will play an additional role in total snowfall
accumulations, with burst of heavy snowfall expected under these
convective signatures. The trough becomes negatively tilted Sunday
morning into Sunday afternoon as a 110 kt jet rounds the base of
the trough through the Great Basin diving south of the Four
Corners. This will enhance the large scale ascent over the region
from southwest (over SE Utah) to northeast (over western
Colorado) from Sunday morning through Sunday evening. As this
happens, a vertically stacked low from the surface up to 500 mb
will develop over east-central Colorado, allowing for wraparound
moisture to move north around the low and into northwest Colorado
by late Sunday afternoon into the evening as the flow switches
around from southwest to northwest. This shows hints of a TROWAL
signature, which could also mean the northwest Colorado valleys
may see some decent snowfall.

With orographics, dynamics, convection and cold air all playing a
factor, this system looks to produce 5 to 10 inches on average
across the mountains of eastern Utah and western Colorado. Some
areas will see locally higher amounts up to 12 inches or a little
more perhaps, with the southern mountains and southwest facing
slopes of the central mountains being favored for the best
accumulations due to strongest ascent over this region as well.
Went ahead and issued Winter Weather Advisories for all mountain
areas with this morning forecast package as models maintain very
good consistency and confidence is high. Travel will also be
significantly impacted if traveling through the higher elevations
and mountain passes.

Will let the day shift further evaluate the lower valleys to see
if any highlites are needed. Overall, thinking the southern
valleys and northern valleys might be favored for the better
valley snowfall. It will all depend on how soon the cold air
arrives to change rain over to snow and keeps p-type all snow as
there appears to be some wavering back and forth. The cold front
also moves through during the daylight hours, which would result
in accumulations mainly on grassy surfaces with just wet to slushy
roads. A couple inches in some valley locations though is not out
of the realm of possibilities.

.LONG TERM...(Sunday night through Friday)
Issued at 458 AM MDT Sat Mar 17 2018

The GFS and ECMWF have a good consensus on moving the storm
center to southeast CO and the Texas and Oklahoma panhandles
Sunday evening while the NAM12 takes the 500 mb low center about
100 miles further north and eventually into west-central Kansas.
A trowal signature is evident in theta-E fields extending north
and west of the low center and over much of western CO through
Sunday evening, bringing the most widespread mountain snowfall
and valley rain and snow through this period. There is potential
for the Uncompahgre Gorge (Highway 550 corridor from Ouray to Red
Mountain Pass in the NW San Juans) to see continued heavy
snowfall late Sunday evening through Monday morning as the flow
switches around to the north and decreases to around 10 to 15 kts,
which is perfect setup for a Gorge event.

Monday should bring northwest flow and lessening moisture over the
area leading to steadily decreasing precipitation. Tuesday and
Wednesday should bring a broad ridge and drier conditions over the
Great Basin and Central Rockies. Then Wednesday night, a branch
of the southern stream will bring moisture north ahead of the next
large Pacific trough. This trough is projected to weaken as it
moves over the Great Basin Thursday through Friday morning,
leading to a drying trend Friday.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 458 AM MDT Sat Mar 17 2018

VFR conditions will prevail through this afternoon and evening
with breezy conditions for all TAF sites. Mountains in northeast
Utah, north of Vernal, will be obscured in snow showers this
afternoon. Conditions will deteriorate late this evening into
Sunday morning as moisture moves in and precipitation increases
across the region, with snow in the mountains and rain in the
lower valleys, changing to snow by Sunday morning.


CO...Winter Weather Advisory from 3 AM Sunday to 9 AM MDT Monday for

     Winter Weather Advisory from 3 AM Sunday to midnight MDT Sunday
     night for COZ003-017.

     Winter Weather Advisory from 3 AM Sunday to noon MDT Monday for

UT...Winter Weather Advisory from 6 PM this evening to midnight MDT
     Sunday night for UTZ025.

     Winter Weather Advisory from noon today to 6 PM MDT Sunday for

     Winter Weather Advisory from midnight tonight to midnight MDT
     Sunday night for UTZ028.



AVIATION...MDA is the U.S. government's official web portal to all federal, state and local government web resources and services.