Intricacies of Medium-Range Monsoon Surge Forecasts

I don't know about you, but I look forward to another rainy day or night and have started to surf through the forecasts looking for hope.  

There is some potential for later this week and weekend, but subtle differences in the large-scale pattern could make a big difference.

To illustrate this, I'll compare the GFS and the ECMWF (a.k.a. Euro) forecasts valid 0000 UTC 19 August (6 PM MDT Friday) and 1200 UTC 21 August (6 AM MDT Monday). 

The GFS forecast for 0000 UTC 19 August (6 PM MDT Friday) shows two key large-scale circulation features: (1) ridge centered over Oklahoma and (2) a trough off the SoCal coast at 500 mb (top left panel below).  Southerly flow between these two features transports moisture from the Gulf of California region up the lower Colorado River basin and into Utah.  

The tricky thing for the Salt Lake City area is that there is a sharp contrast cross northern Utah between air that originates from northern Mexico and the Gulf of California, which is relatively moist, and drier that originates of the California and Baja California coast, which is drier.  This leads to a sharp contrast in 700-mb humidity (lower left panel) and model forecast precipitation (upper right) across northern Utah.  A small shift in position could make a big difference for Salt Lake City.  Note that this situation is not all that unusual during the monsoon.  

The ECWMF forecast is remarkably similar on the large scale, but the moisture contrast is just a bit farther west.  If this were to verify, the odds of showers and thunderstorms in the Salt Lake area would be a bit higher.  

Pushing things out to 1200 UTC 21 August (6 AM MDT Monday), the GFS is positively giddy about a strong monsoon surge.  As a short-wave trough that was located over the Queen Charlottes digs into the western US, it strengthens the moisture transport up the lower Colorado River valley and into western Utah.  There may be some contributions from a tropical cyclone that is expected to develop over the eastern Pacific the next couple of days in the form of a moisture surge up the Gulf of California to supplement things.  

We might even get some severe convective storms from a forecast like that.

The Euro, however, is not as excited for Salt Lake City.  Really, the forecast isn't tremendously different.  It has a trough over the Pacific Northwest swinging in and it has a tropical cyclone in the eastern Pacific.  However, these are both more progressive, meaning that they are moving downstream faster.  This leads to the eastward moving northwest trough being a bit farther inland and the westward moving tropical cyclone just a bit farther offshore.  As a result, northwest Utah is in a drier airstream and the Gulf of Califonia surge is essentially non-existent.  There is still a monsoon surge, but it's not as potent and it may result in the action being a bit farther east. 

These two forecasts illustrate some of the sensitivities of monsoon surges to small differences in the large-scale forecast. In terms of the overall pattern, these forecasts are not that different, but the magnitude of the moisture transport and the location of moisture plumes and precipitation differ, and that matters in this case for the Salt Lake Valley.  

We will see how this all shakes out. 

Meteorological Contributors to the Maui Fires

At least 36 people have been killed by tragic wildfires on Maui.  The NASA/USFS Fire Information Resource Management System shows fires/hotspots around Lahaina and over interior Maui.  

Prior to this event, the National Weather Service was well aware of the potential for strong and damaging trade winds, as evident in the forecast discussion below from 3:53 PM HST Monday August 7.   

They had also issued a red-flag warning for leeward (west) areas on the islands due to low humidity and strong winds with gusts over 60mph.  It was noted that any fires would "spread rapidly."  

I could not find any observations from the Lahaina area through my usual channels.  Instead, I will use data from the Kealia Pond National Wildlife Refuge with is about 20 km to the southeast (and south of the high terrain immediately east of Lahaina.  This difference could be very important as the topography of the Hawaiian Islands has highly localized impacts on airflows.  That said, if we look at a 7-day time series from this location, we see dramatic and important changes occurring on the 7th and 8th.  

Prior to the 7th, temperatures were generally reaching or just below 90°F each afternoon.  Dewpoints were between 60 and 70°F.  Winds would pick up each afternoon, with gusts peaking at about 30 mph.  

On the 7th, temperatures reached 95°F and afternoon winds gusted to as high as 37 mph.  Gusts over 25 mph continued overnight.  During the day on the 8th when the wildfires broke out and spread rapidly, temperatures once again climbed to 95°F, but the dewpoint also dropped to 55°F.  Wind gusts reached 53 mph shortly after noon HST.  

Media reports have mentioned Hurricane Dora.  Dora Passes well to the south of the islands, but it may have important indirect effects.  The analysis below shows the heights (black contours), wind speeds (color contours in meters per second), and winds (full barb = 5 m/s or 10 knots) at 925 mb in the days leading up to the event (the last analysis if 1800 UTC 8 August).  As Dora moves westward, the subtropical high to the north also strengthens and moves westward.  This greatly enhances the easterly trade wind flow approaching Hawaii.  

The GFS poorly resolves the Hawaiian Islands, but the topography of those islands, as discussed in the above NWS discussion, can strongly affect local airflows.  Strong winds at Kealia Pond National Wildlife Refuge likely reflected gap flows between the two primary peaks of Maui.  Numerical simulations may be needed to tease out what happened at Lahaina.

A Trough!

It's always exciting in the dog days of summer to see a trough embedded in the midlatitude westerlies pay a visit to Utah.

The GFS forecast below shows the trough moving through Utah at 1800 UTC (1200 MDT) August 8 (today).  At 700 mb there is even...wait for it...a temperature gradient with cold air advection (i.e., the transport of cold air into the area).  

It's difficult to find evidence of a well-developed front at the surface, but there is evidence of some enhanced northerly or northwesterly flow over the west desert and along portions of the northern Wasatch Front at around 8 AM this morning.  Winds in the Salt Lake Valley, which are often southerly or southeasterly in the morning if there's no large scale forcing, show a bit more disorganized character.   

That could reflect some of the showers that moved through, but also the fact that the surface trough is nearby or just downstream.  

The NWS forecast for today calls for scattered showers this morning and then sunny skies this afternoon with a high of...wait for it...81°F!  

ENJOY!

After the Deluge

Yesterday brought another day of heavy monsoon convection to northern Utah.  The National Weather Service 24-hour precipitation analysis for the period ending at 1200 UTC 4 August (6 AM MDT Friday) shows totals in northern Utah reaching over 5 inches in the area north of Brigham City.  

An observing site near Honeyville near that maximum reported a 48-hour total of 5.83".  There is some concern about the veracity of that sensor, so this will need to be verified, but other stations in that area reported over three inches.  

Meanwhile a bit further south, convection moved across the southern Salt Lake Valley and northern Utah County yesterday evening.  The storms produced a remarkable amount of lightning.  

Well played Mother Nature. Well played. pic.twitter.com/vyogDD4U2j

— Jim Steenburgh (@ProfessorPowder) August 4, 2023

The analysis above indicates maximum accumulations of between 1.5 and 2 inches, although there is one pixel that may be above 2.  A closeup of radar estimated precipitation in the Corner Canyon area shows estimates over 1.8 inches in an hour.  

Draper Mayor Troy Walker declared a state of emergency due to flooding and damaged roads.  The storm intensity was similar to the one that hit the Sugarhouse in July 2017 and put down about 2.28" in 60 minutes (see Another Look at the Deluge...although this was a different deluge!).  It was a monsoon storm producing heavy precipitation in a highly localized area overwhelming the human-built system.  Hopefully the damage is limited.

An Impressive Monsoon Surge

What a day yesterday and what a morning this morning.  

As anticipated, a very juicy airmass moved into Utah this week.  The 5-day time-series from the airport shows the gradual increase of dewpoint and decrease of temperatures from July 29th to August 1st.  Then, dewponts really jumped on the night of the 1st, reaching 65˚F and ultimately hitting 70˚F during the day yesterday.  They have remained at or above 60˚F since about midnight on the 2nd.  

Yesterday, a powerful squall line moved northward through western Utah and ultimately through the Salt Lake Valley in the afternoon.  The loop below shows the system moving northward and illustrates how the National Weather Service tracks these storms and updates warnings accordingly. 

Utahn's don't see this much, but a good example of how @NWSSaltLakeCity continuously tracks and updates severe thunderstorm warnings pic.twitter.com/Egjv1cjh4Y

— Jim Steenburgh (@ProfessorPowder) August 2, 2023

The 70°F dewpoint is probably near the upper end of what you'll see at KSLC, although I don't have a database here to confirm that.  It may be just shy of a record, but I'm speculating.  

Precipitation was fairly widespread in the Salt Lake Valley, with a lot of variability in amounts consistent with the convective nature of the storm.  The highest total reported to the NWS so far is 2.62" in West Valley City (through 4:50 AM).  Lower amounts are generally around 0.2".  

For the standard-time calendar day yesterday (August 2nd), KSLC reported 1.31" of precipitation.  That rates as the 16th highest calendar day total during the July to September monsoon period. 

Source: https://xmacis.rcc-acis.org/

All time, it is a less impressive #38 as there are a number of heavy precipitation events that have occurred during the fall, winter, or spring that also pip it.  Salt Lake City actually has a remarkably divers extreme precipitation climatology with events in every month in the top 50.  

I had a wonderful stroll into the office this morning in light rain with the umbrella raised.  Enjoy this cool, moist weather. 

The Heat Was on in July

Some impressive heat records have fallen from the month of July.  Phoenix had it's hottest month ever.

It's official, this July was the hottest month in Phoenix on record, with an average temperature of 102.7°F. This beats the previous record of 99.1°F set back in August 2020 by 3.6°F. It was certainly a month for the record books.🌡 #azwx pic.twitter.com/AvtODKN00s

— NWS Phoenix (@NWSPhoenix) August 1, 2023

The Washington Post is reporting that is the hottest monthly average temperature ever recorded in a U.S. city, breaking the previous record of 102.2°F set in Lake Havasu City in 1996.  

My daughter lives in Phoenix and was on a flight to Seattle yesterday.  When they landed they asked everyone to lower their shades because it was "hot out." She said the entire flight erupted in laughter.  

A bit further to the north or west, Las Vegas, Kingman, and Needles all set July temperature records.  

2023: Hottest July on Record for 3 Climate Sites...

🥵 Las Vegas, NV
Average Temp: 97.3 (prev. 96.2 in 2010)
🥵 Kingman, AZ
Average Temp: 88.5 (prev. 86.9 in 2003)
🥵 Needles, CA
Average Temp: 101.8 (prev. 100.9 in 2006)#VegasWxRecords #KingmanWxRecords #NeedlesWxRecords

— NWS Las Vegas (@NWSVegas) August 1, 2023

Had enough?  How about Miami?  Warmest month on record there.

July 2023 was officially the warmest month on record at #Miami, with a mean average temp of 86.5°F.

We thought we'd see how this compared to the records at other cities across the US... are you ok, @NWSPhoenix?@NWSChicago @NWSFortWorth @NWSLosAngeles @NWSSeattle @NWSNewYorkNY pic.twitter.com/7JJ4TRRQuH

— NWS Miami (@NWSMiami) August 1, 2023

More to come I'm sure, including from elsewhere around the world.  

Meanwhile in Salt Lake City, the airport came in with an average temperature of 85.3°F.  That's a full two degrees cooler than last year's inferno, but still rates as the 3rd warmest all time.  

Source: NOAA Regional Climate Centers

Since I always get questions about the representativeness of the airport observation, I'll add that July was the 4th warmest on record at the Bountiful Bench site with an average temperature of 80.9°F, which was only 0.8°F cooler than last year.  Records there go back to 1975, but the 14th warmest Julys on record all have occurred since 2000.  

July is a bad month that is getting badder.