Monday, September 15, 2014

Odile...or Not?

Over the past two days, Hurricane Odile has been rumbling northeastward and crossed the southern tip of the Baja Peninsula overnight.  High integrated precipitable water values (contours below) are rumbling up the Gulf of California with and in advance of Odile, posing the potential for another major monsoon surge and related convection over the next couple of days.   

IR and GFS precipitable water analyses from 1500 UTC 13 Sep – 1500 UTC 15 Sep 2014
As with Hurricane Norbert, the long-range forecasts of Odile by the GFS have been super wacky (a technical term).  The forecast from Saturday morning (13 Sep) called for Odile to move westward and be located well off the Baja coast (near the bottom of the plot below) at 1200 UTC Friday 19 Sep.


The forecast from Sunday morning called for Odile to weaken with just some remnants over the northern Baja Peninsula by 1200 UTC Friday 19 Sep.


And now the forecast from this morning calls for Odile to curve hard to the east and be in New Mexico by 1200 UTC Friday 19 Sep.


The latest ECMWF model forecast favors southeast Arizona at 1200 UTC Friday 19 Sep.


Ah, the forecast joy of eastern Pacific hurricanes interacting with midlatitude troughs!  As the computer said in War Games, the only winning move is not to play.

Friday, September 12, 2014

Canyon Winds

Our first canyon wind event of the season is underway this morning.  Unlike big downslope wind events that typically roar along much of the northern Wasatch Front, this morning stronger winds are confined to a few locals including near Weber Canyon (currently gusting to 36 mph in the canyon) and near Centerville (25 mph).

Wind and wind gust observations within one hour of 14:47 UTC (8:47 MDT).  Source: MesoWest.
There are also east winds on the University of Utah Campus and Parley's Canyon.

Wind and wind gust observations within one hour of 14:47 UTC (8:47 MDT).  Source: MesoWest.
The winds at Parley's Canyon increased at a very gradual rate overnight.  The peak gust of 49 mph occurred early this morning.


The cause of these canyons winds is an area of high pressure that slid down the east side of the Rockies and is currently centered in northwest Nebraska.  The resulting pressure gradient is driving easterly flow into northern Utah.


This easterly flow is, however, very shallow.  By the time one gets up to 700-mb (10,000 ft), the flow over the northwest portion of the state is northwesterly.


As a result, this event is confined to some of the deeper canyons (e.g., Weber, Parleys) and areas where the Wasatch Crest to the east is relatively low (e.g., the University of Utah).

The key ingredient missing from this event and preventing it from being a big, widespread downslope windstorm is a closed upper-level low centered near Las Vegas.  Such an upper-level low is needed to drive easterly flow at mountain-top level.

These east winds should slacken later this morning.  

Thursday, September 11, 2014

Insights into Tornado Formation

Source: Markowski and Richardson (2014)
Paul Markowski and Yvette Richardson of Penn State have a great article out in the latest Physics Today entitled "What We Know and Don't Know about Tornado Formation."  I suspect the article may be paywalled, but it should be accessible to anyone on the University of Utah campus or from other institutions that have a suitable license.  I especially encourage atmospheric and other science majors to have a look.

Paul and Yvette are at the forefront of tornado and severe convection research in the United States and are also known as being outstanding teachers.  They work on different problems than I do, but when I can, I always attend their talks.  The are also the authors of the popular textbook Mesoscale Meteorology in Midlatitudes.

Wednesday, September 10, 2014

Tropical Aftermath and UAC Survey

Blue skies prevail over the Wasatch Front this morning, marking an end to our latest flirtation with the tropics.  This weeks monsoon surge produced some incredible flooding and videos, such as the one below from Shane Dukeman in the Virgin River Gorge (Warning: Disturbing scenes and strong language).  The status of the man who disappears with the van is unknown, but officials in Nevada report no deaths.


With the tropical airmass gone, it's time to start thinking about skiing again.  The Utah Avalanche Center (UAC) is currently conducting a survey on avalanche danger ratings.  Please consider taking the two minute survey at at https://www.surveymonkey.com/s/WLHSRHS.  In addition to their usual clientele, they are especially interested in hearing from individuals who are not already familiar with their danger ratings.

Tuesday, September 9, 2014

Lessons from a Feast or Famine Precipitation Climate

The climate of the desert southwest is characterized by long stretches of monotony broken by periods of intense excitement.

Intense excitement prevailed yesterday as heavy rains brought flooding to locales such as Palm Springs (CA), Las Vegas (NV), the Virgin River Gorge (UT), and Phoenix (AZ).  Given the large spatial contrasts that exist in monsoon precipitation, the event was about as widespread as a monsoon precipitation event can get.

Phoenix has gotten a great deal of attention and for good reason.  The Phoenix Airport recorded 3.29" of rain for the day, all falling in about 7 hours.  Accumulations of over 2 inches covered a large portion of the metropolitan area with some locations reporting more than 5 inches.

Source: NWS
The time series of accumulated average (brown line) and actual (green line) precipitation since the start of the water year (October 1, 2013) shows the feast or famine nature of the Arizona climate.  Average looks nice and smooth, but in any given year, a few storms produce most of the precipitation.  Indeed, you can see three major cool season events, a couple of small monsoon events in August, and then yesterday's whopper add up to an "above average" water year at this location.  


This is a key characteristic of the climate of the southwest.  A handful of big events (or the lack thereof) make or break the accumulation in any given year.  

The 3.29" represents the largest accumulation on any calendar day at the Phoenix airport.  Records go back to 1895, so that sounds impressive, but remember that precipitation is rare in this region, so we're dealing with what scientists call the statistics of small numbers.  In addition, calendar day records can be a bit deceiving.  The record 24-hour period rainfall at the Phoneix airport, for example, is 4.98".  Of course, much of the rain that fell yesterday fell in < 8 hours, so ultimately meteorologists will need to take a broader look at this event to determine where it lies in the spectrum of extreme rainfall events over southern Arizona.   There are a number of good studies in the literature examining this issue, but I lack the time to dig into them before running to class!  Perhaps others can comment.

Meanwhile, the University of Utah has been getting pounded this morning.  Our site near Red Butte Gardens has recorded about 0.35" of rain in 20 minutes.  Students are looking wet!


The 24-h accumulation is 1.39", more than the monthly average for September.  Even northern Utah can get in on the feast-or-famine climate action.  


Monday, September 8, 2014

Phoenix Gets a Pounding

Someone was going to get a beating from the ongoing monsoon surge and right now it is Phoenix.  Observations from the Phoenix International Airport (KPHX) show that 3.09" of rain has fallen since midnight.

Source: NWS
The NWS has issued flash flood warnings for much of the area.
Source: NWS
The 1200 UTC (0600 MDT) analysis below shows the mix of airstreams feeding the storm system, known as a mesoscale convective system, this morning.  Potentially contributing to this event are moisture sources associated with Tropical Cyclone Norbert, a Gulf of California surge, and even the Gulf of Mexico, although the juiciest air appears to be originating from the Pacific and the Gulf of California.


The potential for thunderstorms that could be locally heavy exists across much of Utah today through tomorrow.  Keep an eye to the sky.  NWS forecasts at http://www.wrh.noaa.gov/slc/.