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Area Forecast Discussion (AFD)

Issued by Denver/Boulder, CO (BOU)

                            
000
FXUS65 KBOU 200216
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
816 PM MDT Sun May 19 2019

.UPDATE...
Issued at 816 PM MDT Sun May 19 2019

Main area of showers has lifted north of the Denver metro area. 
The airmass will remain moisture through the night. It won't take 
much lift to produce additional showers. If additional showers 
form overnight they should be light. Still on track for lowering 
clouds and better chances for drizzle and rain showers Monday 
morning. Snow level is running 8000-8500 feet MSL. Not seeing any 
snow on the roadways. With snow expected to decrease overnight 
will keep the start time of the Warnings as is. Going forecast 
appears to be in good shape and will only make minor adjustments 
to line up with current trends. 

&&

.SHORT TERM...(This evening through Monday)
Issued at 334 PM MDT Sun May 19 2019

Current satellite shows shallow convection is becoming more
widespread in the high country as expected. Instability is 
limited, however, as indicated by some trapped waves in the 
satellite imagery. Thus, initial precipitation may have a hard 
time moving off the Front Range. Best chance would be along and 
north of the upper level jet axis, which is streaking northeast 
along the I-76 Corridor. There is even some transverse banding 
(indicating intensity of the speed max) noted on the satellite 
imagery, and this along with cooling cloud tops are racing toward 
the forecast area. The existing forecast has the highest chances 
over the mountains and northern border area, and this looks good 
through the evening based on the above factors. Snow level appears
to be shaping up at 9000-9500 feet, and the 2-5 inch forecast for
late this afternoon through the evening looks good, favoring the 
mountains north of I-70 for the heavier amounts. 

Precipitation coverage may decrease slightly later this evening, 
but another round of increasing QG lift on the nose of a stronger 
upper level speed max arrives late tonight and tomorrow morning. 
With strong easterly flow across the plains, there will also be an
increase in at least drizzle and probably showers as moisture 
deepens with the synoptic lift. Precipitation coverage will 
continue to expand and increase in intensity through the day 
Monday as the strengthening QG lift and jet dynamics combine with 
a neutral to slightly unstable airmass. A few ensemble members 
have a narrow ribbon of MLCAPE near 500 J/kg over the Palmer 
Divide, but at this time would lean toward less CAPE and the NAM 
which shows much cooler air holding across the plains through the 
day. As a result, think the threat of any stronger storms Monday 
afternoon would be rather limited and confined to points to our 
south. Still could see a couple hail producers from elevated 
convection. 

Finally, mountain snowfall should see a significant uptick with
the arrival of better dynamics and heavier precipitation rates.
We'll begin the Winter Storm Warning for the high country at Noon,
with much more snow to come through Monday night and Tuesday...see
Long Term discussion below. 

.LONG TERM...(Monday night through Sunday)
Issued at 334 PM MDT Sun May 19 2019

The storm system responsible for precipitation on Monday looks to
finish digging south by Monday night, then intensify as it lifts 
out to the northeast during the day on Tuesday. By sunrise 
Wednesday the vorticity maximum should be located in the Central 
Plains of NE/SD, and the lift and moisture will go with it for 
all but the mountains. Between Monday night and early Wednesday 
there will be a significant precipitation event for most of our 
area, along with potential impacts to travel through the 
mountains. Despite the event being 30-60 hours from now, there 
are still a few sources of uncertainty even at the synoptic scale.
First, it is noteworthy that the GFS, ECMWF, and GEM are all in 
fairly good agreement on the timing, intensity, and position of 
the trough as it moves from the four corners region late Monday, 
then across eastern Colorado into the plain. There are significant
differences in the temperatures and QPF. The NAM is the coldest, 
the GFS is the warmest, and the ECMWF is in between but closer to 
the NAM than to the GFS. Minor differences in position, especially
of the 700 mb low, will determine who gets the highest QPF. Based
on recent trends, we feel the 700 mb low will track from around 
the San Luis Valley to ~Bent County to around Goodland, KS by 
midday Tuesday. The surface low will follow a similar track, 
deepening 20-22 mb from Monday 12Z into Tuesday afternoon, a 36 
hour period of deepening. QG lift from the 500 mb trough is 
strongest through midday Tuesday, and as it becomes negatively 
tilted a jet streak will likely generate additional lift across 
the northeast third of Colorado located under the left exit 
region. Finally, east to east-southeast upslope flow will be 
strong through sunrise Tuesday before the low pulls out to the 
northeast. With strong integrated vapor transport (3-6 standard 
deviations into Eastern Colorado), precipitation generation should
be easy for the atmosphere across out area.

Looking at the mesoscale for more details and potential impacts, 
best best for now is that the snow level will remain above 6,000 
feet MSL. It gets close to population centers around sunrise 
Tuesday morning and if precipitation is still going strong above 
5,000 feet, there is an outside chance parts of metro Denver could see
snow though accumulations would be negligible. The NAM develops 
cold air damming early Tuesday, funneling cold air from Wyoming 
along the front range west of US85, and lowing snow levels to the 
surface. That type of set up usually requires intense snow aloft 
and strong easterly upslope flow, but for now feel the NAM is 
overdoing the intensity of the precipitation, the strength of the
cold air in general, and thus the cold air damming. We'll be 
closely monitoring the other models to see what they do because 
under that scenario accumulating snow would be possible for parts
of metro Denver. Aside from the NAM, the rest of model guidance 
keep accumulating snow in the foothills above 7,000 feet. 

The models are not consistent regarding QPF, but are trending to
favor the northern foothills, Cheyenne Ridge, and the northern 
row of Colorado Counties. This includes the Fort Collins-Loveland-
Greeley metro area. An inch to an inch and a half of rain is 
possible across the lower elevations from Larimer County east to 
the Nebraska border, and in the northern foothills and mountains 
above 7,000 feet should get clobbered with snow. We have a WSW out
for the northern foothills of Boulder and Larimer Counties for
8-18 inches of snow. More uncertainty exists for the foothills 
below 9,000 feet for Jefferson/Gilpin/Clear Creek/Park Counties 
where downslope could impact amounts and the best upslope is north
of there. We left those areas out of any highlights but still 
expect 2-5 inches above 7,000 feet for those areas however. The 
central mountains and Summit County should also fare quite well 
in terms of snow, with warning criteria snowfall expected through
Tuesday. Travel will be challenging through the mountains from 
Genesee to Vail Pass along I-70, and across the higher passes 
like Cameron, Berthoud, Rabbit Ears and others above 9,000 feet 
across our area through Tuesday and overnight into Wednesday. The 
saving grace could be that during the day Tuesday the high sun 
angle should keep the roads warm, although in the face of heavy 
snow roads would still be slick during the day.

A bit of uncertainty exists for the higher elevations of the
Palmer Divide regarding snow potential. The highest elevations 
near Monument could see several inches of snow on the grass, but 
if temperatures and low-level flow falls in line with the NAM, 
highlights could be needed for the Palmer Divide given the 
strength of the potential upslope in that scenario. Confidence is 
far too low too for a highlight at this time. The good news for 
those that like precipitation is that nearly all of our area 
should see a good rain across the plains and accumulating snow 
across the higher elevations.

Northwest downslope off the Rockies kicks in as the surface and
700mb low pressure centers move northeast of the state Tuesday
afternoon. Precipitation will wind down first along the Front 
Range urban corridor Tuesday afternoon/evening, then spread east.
Precipitation across the rest of the plains should shut off by early
Wednesday morning. The mountains will continue to see up to 
moderate snow on the backside of the trough Tuesday evening into 
Wednesday via west upslope, and thus the reason for keeping the 
WSW through midnight. Light snow could continue into late morning 
Wednesday across the northern mountains but additional 
accumulations and impacts should hopefully be minimal.

Southwest flow continues later this week with yet another 
upstream trough. Wednesday afternoon should be mostly dry aside 
from a chance of afternoon convection in the high country spilling
east across the plains, but coverage and intensity are not 
expected to be extensive. Highs should remain below normal 
Wednesday and Thursday. The main lift from that trough should 
remain north and west of us but there is a decent chance of 
afternoon convection especially across the northern third of 
Colorado. Shear profiles would be favorable for severe weather but
low-level moisture is lacking, so instability will most likely be
limited. If low-level moisture can make it west into our area 
this will be an afternoon to watch.

Friday looks dry across our entire area and warm advection with
drier southwest flow should warm temps back to near normal with
highs in the low 70s across the Plains and upper 40s to around 60
in the mountains. The warmer near normal temperatures continue 
into the weekend but afternoon thunderstorms are a decent 
possibility each afternoon Saturday and beyond, initiating in the
mountains and moving east.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 816 PM MDT Sun May 19 2019

Denver convergence zone continues to wiggle back and forth 
causing wind shifts. The next wind shift looks to be around 0230Z 
as winds shift back to the east-northeast. Chances for additional 
rain showers through the evening is low. Lower MVFR/IFR ceilings 
are expected to redevelop after 06Z tonight with a few showers in 
the vicinity. Lower ceilings will likely hold around through all 
of Monday with moist north/northeast boundary layer flow and 
increasing showers and thunderstorms through the day. Heavier 
showers and storms after 18Z will likely produce limited 
visibility of 2-3SM in brief periods of moderate to heavy rain.

One last note, if the Denver convergence zone does wash out 
tonight or Monday morning, then we would have a chance of stronger
easterly winds with lower clouds, drizzle and fog. Right now that
threat looks to be about 20-30%. 

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from 6 PM Monday to midnight MDT Tuesday 
night for COZ035.

Winter Storm Warning from noon Monday to midnight MDT Tuesday 
night for COZ031-033-034.

&&

$$

UPDATE...Meier
SHORT TERM...Barjenbruch
LONG TERM...Schlatter
AVIATION...Meier