Day 24: Winter Storm Culprit

Posted by pmarsh on 24 Jan 2010 | Comments Off

Numerical weather prediction models are in fairly good agreement that there is a potential for a significant winter storm across the southern portion of the United States mid-to-late week.  As I indicated yesterday, one of the main ingredients necessary for a winter storm (cold air) should be in place.  What about the other two, moisture and a source of lift?  Well, we’ll leave the moisture question for another night, but the source of lift can be identified looking at satellite imagery of the pacific.

The white areas circled in yellow (image below) are associated with a mid-level shortwave trough.  Shortwave troughs are often described (incorrectly) by meteorologists as a “disturbance”, “impulse”, or “piece of energy”.  As you can see, this shortwave trough is still way out in the pacific and the exact track of it is still anyone’s (or any model’s) guess.  Currently, most medium range forecast models indicate that the shortwave trough will track across northern Texas into southern Arkansas.  On this track, a good portion of Oklahoma and portions of northern Arkansas will be favored for some sort of wintry precipitation.  At this point, it is too far in the future to pinpoint what kind and how much wintry precipitation will fall, but people in the southern plains should begin to prepare for a potential winter storm and pay close attention to forecasts over the coming days.

Day 23: Is the Cold Air Returning?

Posted by pmarsh on 23 Jan 2010 | Comments Off

The National Weather Service’s Climate Prediction Center (CPC) produces “climate” forecasts on various time scales ranging from 6-10 days all the way to the next three months.  These predictions are rather generic in nature, essentially providing a prediction as to whether temperatures will be above normal, equal chances of above or below, or below normal.  They do the same for precipitation.

The forecast below was issued by the CPC on 23 January and is valid for the time period of 29 January – 2 February.  It is an example of their 6-10 day outlooks.  What is impressive about this forecast is the vast area that is expected to have at least a 33% chance of having colder than normal temperatures in that time frame (this is the blue shading).  Most of this same area is expected to have greater than 50% chance of having below normal temperatures, and the south central United States (Texas area) is forecast to have a greater than 60% chance of having below normal temperatures.  With values like this forecast, there must be a pretty strong signal that cold air is coming.

So how can meteorologists offer such a high probability forecast so far in advance.  Well, for one, the numerical weather prediction models (NWP) must be in fairly good agreement that something like this is possible.  Another technique, and the one I advocate strongly to my students, is to take a look at observations and see if the signal is already there.

The image above is the current 00Z (6 PM CST) North American surface map.  A couple of things stand out.  First, there are several big red “L”s in the middle of the map connected by a blue line with barbs on it.  These big red “L”s are where the surface pressure is relative minimum, or, in other words, lower than all surrounding areas.  This is what meteorologists are referring to when they say “low pressure”.  These low pressures and the associated cold front(s) (depicted by the blue lines with blue barbs) are responsible for the precipitation in last night’s image.

Of interest for this post is the blue H (a relative maximum in surface pressure, or, in other words, the point where the pressure is higher than all surrounding areas) in northwest Canada, near the Alaskan border.  This high pressure is associated with cold air in northwest Canada, that is poised to filter southward.  How do I know this?

Well, take a look at the reddish, brown lines that encircle the L’s and H’s through the map.  These lines are known as “isobars”, or lines of constant pressure.  If you look at the lines encompassing the high pressure in northwest Canada, you can see how there are distinct “kinks” in the circle that stretch from the H, all the way southward to the cold front (blue line with barbs). (At this point I should mention that the image below is identical to the image above.  The difference is that I’ve circled the high pressure in yellow and drawn an arrow through all the kinks in the isobars in pink.)  Believe it or not, but the high pressure in Canada is affecting the cold front that is moving through Kansas, right now!  Over the next week, this cold airmass in Canada will initially filter southward along the path laid out by the kinks in the isobars to the south of the high pressure.

Why am I so confident this will happen?  One reason is that there are no fronts (cold, warm, or stationary) between the cold front in Kansas and the high pressure in Canada.  Fronts are boundaries in the atmosphere.  In fact, the reason meteorologists call these boundaries “fronts” is because in warfare the boundary where two armies fought is known as a front.  Well, in the atmosphere, when warm air and cold air meet, the boundary is known as a front.  Because there is no front between the high pressure in Canada (associated with arctic air) and the front in Kansas, there isn’t really anything in the atmosphere to stop the cold air from moving southward – especially on the east side of the high pressure.

It will take several days for the brunt of the cold air to make it into the United States.  However, as it does, NWP models indicate that a storm will move out of the southwest United States into the central plains.  Depending on how long it takes the cold air to become entrenched in the US (and how far south it makes it), portions of the central United States could be looking at a major winter storm middle-to-late this week as far south as Oklahoma.  Freezing rain, sleet, and snow will be possible in the cold sector (cold air), and severe thunderstorms will be possible in the warm sector (warm air).  I’m sure I will be blogging about this more as the event gets closer.

The two images below are of the same mentality as the pictures above.  The only difference is the images are much larger and have more surface observations on them.  I encourage you to click on the images and take a look at the cold air near the high in Canada.  That’s the cold air that will be coming south in the next week or two.

(Note, once you click on an image, if you click on the green arrow in the bottom center, the image will appear in full resolution.)

Day 22: The Rains Head East

Posted by pmarsh on 22 Jan 2010 | Comments Off

The large, powerful storm that has been affecting the west coast for the past few days is beginning to move east into the central plains.  This storm system has tapped into enough warm air in the lower portion of the atmosphere that most of the precipitation is falling in the form of rain (green).   In fact, outside of the higher elevations in the west, you have to go up to the US – Canadian border to find any mixed (orange) precipitation or snow (blue).  This storm system will slowly move east over the next few days and, in its wake, colder air will begin to filter south across the central United States.  Don’t worry, the colder air won’t be nearly as cold as it was earlier this month!  (At least initially…)

Day 21: More on the West Coast Deluge

Posted by pmarsh on 21 Jan 2010 | Comments Off

Over the past week, a series of storm systems have walloped the west coast of the United States with wind, rain, and even a rare southern California tornado!  So much rain has fallen in such a short amount of time, mudslides are becoming a worry.  Today’s set of  images attempts to put into context just how much rain has fallen in southern California in terms of the last six months rainfall total.  Please note that today’s rain is *not* included in these totals!

Also, continuing with the tornado watch theme from yesterday, a rare tornado watch for southeastern California and southern Arizona was issued by the Storm Prediction Center earlier today.  This is the eighth tornado watch this year (also the eighth tornado watch issued in the last 36 hours!).

The image above displays the “normal” expected rainfall for 14-21 January.

The image above displays the observed rainfall for 14-21 January of this year.

The image above displays the departure from normal rainfall for 14-21 January 2010 in terms of inches.  Several places have received more than 8 inches more than normal.

The image above displays the departure from normal rainfall for 14-21 January 2010 in terms of percent of “normal”.  Several places have received more than 600 percent of their normal rainfall.

The image above displays the departure from normal rainfall for the past 180 days, ending 21 January in terms of inches.  Even with the heavy rains of the last week, most places in the desert southwest still remain several inches below normal.  Places that are not significantly below normal are typically within 2 inches of normal.  When you consider how much rain has fallen over the past week, one begins to appreciate the drought that was plaguing the southwest.

The image above displays the departure from normal rainfall for the past 180 days, ending 21 January in terms of percent of normal.  Even though a lot of places are within 2 inches of normal, they have received only about half to maybe three-quarters of their normal rainfall.  This goes to show just how little rain the southwest receives – they can be within two inches of normal and still only have 50% of what they would expect.

Also, not everywhere in the last two images have below average rainfall over the last 180 days.  Places along the coasts and in the higher elevations of California are above normal – however, a significant portion of that has fallen this week!

And last, but not least, the outline and (color-filled) counties included in the desert southwest tornado watch!

Day 20: First Tornado Watch

Posted by pmarsh on 20 Jan 2010 | Comments Off

As was discussed in yesterday’s post, the wave train of short-wave troughs (upper-level lows) has aided in the development of an active weather pattern.  Overnight into this morning, portions of Iowa received over an inch of ice accumulation as the result of freezing rain.  At last check, 20,000+ customers were without power throughout Iowa.

To the south, severe thunderstorms and tornadoes were developing across the southern United States, in what is known as the warm sector (A region of warm surface air between a cold front and a warm front.).  The first tornado watch of the year was issued today by the Storm Prediction Center (SPC).  For those who have never seen the text of a tornado watch, I’ve included the text for this watch below.

SEL1

   URGENT - IMMEDIATE BROADCAST REQUESTED
   TORNADO WATCH NUMBER 1
   NWS STORM PREDICTION CENTER NORMAN OK
   315 PM CST WED JAN 20 2010

   THE NWS STORM PREDICTION CENTER HAS ISSUED A
   TORNADO WATCH FOR PORTIONS OF 

          PARTS OF LOUISIANA
          SOUTHWEST MISSISSIPPI
          SOUTHEAST TEXAS

   EFFECTIVE THIS WEDNESDAY AFTERNOON AND EVENING FROM 315 PM UNTIL
   1000 PM CST.

   TORNADOES...HAIL TO 1.5 INCHES IN DIAMETER...THUNDERSTORM WIND
   GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE
   AREAS.

   THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 55 STATUTE
   MILES NORTH AND SOUTH OF A LINE FROM 80 MILES WEST OF FORT POLK
   LOUISIANA TO 60 MILES EAST OF MC COMB MISSISSIPPI.  FOR A
   COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE
   UPDATE (WOUS64 KWNS WOU1).

   REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR
   TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH
   AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR
   THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS
   AND POSSIBLE WARNINGS.

   DISCUSSION...TSTMS SHOULD CONTINUE TO INCREASE IN COVERAGE AND
   INTENSITY THIS AFTERNOON INTO EVENING...DRIVEN BY BROAD LOW-LEVEL
   WAA REGIME.  AMBIENT AIR MASS HAS BECOME WARM...MOIST AND MODERATELY
   UNSTABLE WITH MLCAPE OF 1000-2000 J/KG.  WHEN COUPLED WITH 40-45 KT
   OF DEEP-LAYER LAYER AND STEADILY STRENGTHENING LOW-LEVEL SHEAR ALONG
   LLJ AXIS...ENVIRONMENT WILL FAVOR THE DEVELOPMENT OF SUPERCELLS
   CAPABLE OF A FEW TORNADOES...IN ADDITION TO HAIL AND LOCALLY
   DAMAGING WIND GUSTS.

   AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL
   SURFACE AND ALOFT TO 1.5 INCHES. EXTREME TURBULENCE AND SURFACE
   WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO
   450. MEAN STORM MOTION VECTOR 24035.

   ...MEAD/WEISS

Shortly after issuing the first tornado watch, tornado watch number two was issued to the northwest of tornado watch number 1.  It is certainly shaping up to be an active evening and overnight across the southern United States.  As the severe thunderstorm even unfolds, I’ll attempt to provide reports that I’ve heard about via my Twitter account.

Day 19: Riding the Wave Train

Posted by pmarsh on 19 Jan 2010 | Comments Off

After last week was relatively quiet, albeit cold in places, this week is promising to be very active in terms of low pressures, upper-level lows, and precipitation.  Just look at the 5-day precipitation forecasts from the Hydrometeorological Prediction Center posted from a few days ago! One reason for the wet, stormy weather pattern is the number of storm systems (an organized collection of low pressure/height throughout the atmosphere) that are identifiable on water vapor.

In the image below, five upper-level lows are circled – two in yellow and three in red.  I’ve used two colors to distinguish between upper-level lows that have aided in the recent west coast deluge (red) and those that haven’t (yellow).  (Author’s note: It is possible to identify additional upper-level lows, vorticity maximums, and/or short-wave troughs; however, I only circled those “easily identifiable” on a single image – not a loop.)  When analyzed on weather maps, upper-level lows often look like waves with crests (ridges) and troughs (troughs).  When several of these waves line up one after another, similar to box-cars on  a train, meteorologists call it a wave train.  Hence the name of this post.

The eastern (rightmost) upper-low circled in red is currently aiding the development of showers and thunderstorms across the southern plains and will affect a large portion of the central United States on Wednesday.  In fact, Ice Storm Warnings are in effect for a good portion of Iowa and a chance of severe weather is possible across a large portion of the north central Gulf Coast states.

The middle of the upper-lows circled in red moved onshore in southern California this afternoon and resulted in at least three tornado reports along the California coast.  The western (leftmost) upper-low circled in red also came ashore earlier today with a myriad of severe weather associated with it.  I’m uncertainly, but one of these two upper-lows was responsible for a wind gust of 93 mph recorded at Newport Beach Pier.

The parent long-wave trough (giant upper-low) responsible for these short-wave troughs (smaller upper-lows) will remain off the west coast for the next several days.  When this long-wave trough begins to move eastward, a decent severe thunderstorm threat will be possible across a large portion of the southern United States.

Day 18: And the Rains Came

Posted by pmarsh on 18 Jan 2010 | Comments Off

The much advertised rains have begun to pummel southern California.  Radar estimates (which tend to be low because of the many mountains and high bluffs) of over 7 inches of rain are already showing up southeast of Santa Barbara in Ventura county.  Remember the big “Station Fire” from 2009?  Several mudslides have already been reported in the charred areas left over from the fire.  You can check out more news results regarding the  southern California storm along with discussions about its impact on the areas impacted by the Station Fire here.  This is just day 1 of a prolonged rain event and several residents have already been asked to evacuate.