A BUCKETFUL OF WEATHER

terryswails1
20 hours ago
6 min read

BACK TO BACK STORM SYSTEMS THURSDAY AND SATURDAY

A very energetic storm track is set to deliver 2 disturbances the next 72 hours that results in a wide array of weather ranging from thunderstorms to light snow showers and most everything in between. A healthy rain is likely Thursday in my northwest counties, and some snow showers are still a threat in most areas Saturday, although that concern has lessened considerably. Throw in a 30 degree drop in temperatures, and you've got yourself a bucketful of weather! So, buckle up...

The two distinct systems are evident on the GOES West satellite imagery. Look at the subtropical jet pouring into the Southwest, spinning up the initial storm and its rain that arrives by Thursday afternoon. Beyond that, more energy is diving into the Pacific Northwest. It digs into the SW where it ejects Saturday and joins forcing with the northern (polar) jet. The merger allows enough cold air to bring any precipitation that falls Saturday to be in the form of snow. Phasing is a complex interaction of energy, something that models are known to struggle with, especially 3 or more days out. The GFS has recently been a bit more aggressive with the process than the EURO, meaning more in the way of the way light snow or snow showers (perhaps an inch or so, mainly east of the Mississippi). The EURO has really backed off and gets the merger done later and further east, resulting in minor accumulations locally that are generally well less than an inch, if that. More on that part of the story in a bit.

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THE DOUBLE SHOT...

Storm one is notable for the amount of moisture it contains. Available water vapor is jacked at 1.1 to 1.4 inches Thursday afternoon. That's close to the 99th percentile due to the fact that cold air typically rules in January, limiting the amount of water vapor that can reach as far north as Iowa.

The 1.33 inches shown in Dubuque is 6 standard deviations above normal.

For more perspective, that's 450 percent above climatology. A juicy air mass, as we say in the business.

By Thursday afternoon, a dynamic surface low interacts with the moisture to produce a healthy rain shield that blasts rapidly through the region starting in the morning across the south, reaching north of I-80 by early afternoon.

This simulated radar off the HRRR shows the region engulfed in the rain plume at 2:00pm Thursday.

If it were a couple of months later, this could have been a significant severe weather event due to the amount of low level shear. 0 to1 helicity values reflect that from the ground up to just over a half mile, showing significant veering profiles ahead of the low and its warm front.

This virtual sounding for the Quad Cities depicts how winds rapidly change from the southeast to the southwest in this shallow half mile zone Thursday afternoon.

In essence, we have exceptional moisture, and strong veering winds, the type of set-up you look for in a tornadic event that includes supercells. The one missing ingredient is warm air. At best readings in the far south should not exceed 60 which limits instability. CAPE, (convective allowing potential energy) is just 101 j/kg. Thus, we are left with a low CAPE, high shear environment. That should limit the strength of any thunderstorms, but just a bit more heating than anticipated could allow a couple stronger updrafts in the far southeast when the triple point passes mid to late afternoon.

Despite the limited CAPE, the K index which measures low level moisture and changes in vertical temperature structure (measured in lapse rates) exceed 30 and that signifies a respectable potential for scattered thunderstorms, some with heavy rain. The south stands the best chance for storms, but some rumbles of thunder could be heard just about anywhere.

Regarding rainfall, the axis of heaviest rain has continued to inch west due to trends showing a stronger surface low that tracks across eastern Iowa. It now appears that a dry slot will cut through the heart of my area close to the track of the actual low pressure. Heavier amounts will be found further NW in the deformation zone as well as to the SE where thunderstorms are more likely in the warm sector. See if you can find the dry slot in the rainfall forecasts below.

The HRRR

The 3K NAM

The EURO

The GFS

From the look of things, rain should quickly depart late in the day west and early in the evening in the east as the circulation center races northeast. Highs in most areas should hit the low to mid 50s, with a 60 possible in the southeast. It will take most of the day for the warmth to reach my northern counties, with 50s arriving around sunset. This is what the EURO shows for Thursday's warmest readings. Notice the sharp decline in temperatures in the NW half of Iowa, which remains in the cool sector of the storm for its duration.

Winds could also become a factor once again with the passage of the storm center and its cold front, allowing rapidly rising pressures late afternoon or evening. The EURO is really bullish, showing potential gusts of 40-45 mph.

Fortunately, the winds will subside pretty quickly Friday as a weak ridge passes ahead of the next system. Expect a dry day, but a cooler one, with highs back around 40 north to 45 in the south.

STORM NUMBER TWO, SATURDAY...

System number two for Saturday looks far less impressive than it did the previous 24 hours. The phasing of the northern and southern branches takes place later and a little further north. While it does happen, it allows the surface low to form far enough east to provide just a glancing blow to mainly my eastern counties. The deformation band is also a bit weaker, even in my eastern counties, so I'm not seeing major impacts locally. In fact, with ground temperatures as warm as they are, what accumulations do occur could end up more on grassy and elevated surfaces than roads if trends hold.

Here's the surface depiction of the snow swath on the GFS at noon Saturday, showing the band covering all of western Illinois and just catching EC Iowa.

I'm still a bit uneasy about pegging snow totals, but it seems the worst case scenario would be 1 to perhaps 2 inches Saturday into the evening hours. The best case is less than an inch, and that seems to be the direction we are headed. I think tomorrow we should get a firm handle on trends once the initial storm lays down boundaries on its backside. As for now, models are still not tightly clustered and here's what they are suggesting for snow totals.

The GFS, the most aggressive and probably well overdone.

The EURO, far lighter, reflecting less phasing. A trend with increasing merit.

The 3k NAM, a full on nothingburger

However, it plays out, once the phasing reaches its peak and any light snow is over, a potent closed 500mb low drops into northeast Iowa Saturday night. That kicks up the winds and sends a fast moving but intense burst of cold air into the region.

Winds of 35-40 mph later Saturday night will drive the cold air southeast, and early Sunday temperatures will be in the upper teens to low 20s. A long way from the 50-60 degree highs of Thursday. Throw the winds into the mix, and wind chills could slip a bit below zero in the north.

Winds should slowly ease as the day wears on but make no mistake about it, we close out the weekend Sunday with a cold day and highs in the mid to upper 20s along with a noticeable wind chill.

Some slight moderation is likely Monday, but northwest flow gets quickly established thereafter and colder weather quickly returns. At this point, I don't see much for rain or snow chances the balance of next week. If we do, it would be associated with some sort of clipper and should be short-lived and light.

Well, that's about all I can put into this summation. Keep the umbrella handy and enjoy the mild temperatures that will surge in during the afternoon and evening. Nothing like a little spring preview to improve the constitution on a January day. Roll weather...TS