Weather Service Report


027 
FXUS65 KTWC 200338
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
838 PM MST Thu Jan 19 2017

.SYNOPSIS...A series of storms will spread valley rain,
significant mountain snow and gusty winds through Arizona tonight
into Saturday. Snow levels will lower through that time, potentially
reaching the valley floors east of Tucson Saturday. After a break
in the weather on Sunday, another storm system is likely on Monday
and Tuesday.

&&

.DISCUSSION...Our 00z KTWC sounding is showing precipitable water
around 1/2 inch with plenty of moisture around 600-700hPa, but a
relatively dry subcloud layer. We are picking up some virga across
SE AZ, but as of yet not much more than sprinkles in valley
locations. We will pick up more shower activity in the early
morning hours, but best chances will probably remain in northern
areas as the first trough inflection pushes through Arizona
tonight. Increasing dynamics and upslope effects ahead of the main
storm will see showers increase tomorrow with most favored period
still expected to be Friday night into Saturday morning as the
main energy and coldest air pushes in from the northwest.

A couple of modest adjustments to the forecast based on 00z
analysis and model data:

We added Eastern Pima (including Tucson) as well as South Central
and Southeast Pinal county to the wind advisory associated with
the strong cold front Friday night.

We have adjusted snow ratios associated with the main system up
just a bit, reflecting more favorable saturation vapor pressures
for more rapid snow crystal growth. The end result adds an
additional 1-4 inches to mountain snowfall amounts Friday night.

Please see the previous discussion below for additional details.

&&

.AVIATION...Valid through 21/06Z.
VFR overnight with BKN/OVC mid clouds and a few -SHRA overnight.
More persistent areas of showers should arrive between 11Z-15Z
Friday with cigs dropping to around 3-4kft AGL. Brief MVFR
conditions possible with the increasing shower activity. Aviation
discussion not updated for TAF amendments.

&&

.FIRE WEATHER...A series of storms will affect the region over the
next week. Gusty west to southwest winds are expected at times
through this period especially Friday night and across the
mountains. Dry weather returns Sunday before another storm affects
the area Monday into Tuesday.

&&

.PREV DISCUSSION...Upper dynamics increases over the area tonight
as the first of two upper level trofs moves across the area.
Decent orographics will bring occasional moderate snow tonight
across the northern mountain ranges which includes the Catalinas,
Rincons, Pinalenos and up to the White Mountains. Winter storm
warning goes into effect later this evening for these ranges with
snow amounts on the low end of warning criteria.

Main event with stronger/colder/wetter upper level trof still on
track for Friday afternoon into Saturday. Very impressive
orographics will lead to impressive snowfall rates Friday evening
into Saturday morning with 1 to 2 feet occurring in some of the
mountain ranges. UofA WRF runs of the NAM and GFS for the past
several days have been rather robust in producing snow amounts
over 4' for the Catalina and Pinaleno mountains at the highest
elevations. WOW!.

In any event, expect significant snowfall amounts above 5500 feet
for this event with localized spots near 3'. Snow levels fall as low
as 4000 feet by Saturday morning.

Rainfall amounts for this event will range from 0.50" to 1.75" with
localized amounts over 2". 

Strong gusty winds will be also part of this storm Friday afternoon
into Saturday morning. We went ahead and hoisted up a wind advisory
for the lower valleys west, south and east of Tucson Friday night
into Saturday morning. Tucson will be close to advisory criteria
Friday night and may end up eventually added to the advisory. In the
mountains it will be howling with the combination of wind and
falling snow producing localized blizzard conditions. This is
handled in the winter storm warning.

Drier conditions Sunday before another Pacific storm impacts the
area Monday and Tuesday with another round of valley rain and
mountain snow. 

High temperatures for the next 7 days will remain below normal with
the coolest days being tomorrow and Saturday. Could be quite chilly
Sunday morning in wake of upcoming storm with eastern valleys seeing
low temperatures in the 20s.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until noon MST Saturday for AZZ510-511-514.

Wind Advisory from 8 PM Friday to 8 AM MST Saturday for 
AZZ501>508.

Winter Storm Warning from 5 PM Friday to noon MST Saturday for 
AZZ512-513-515.

&&

$$

Meyer

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