WE'RE ON THE WAVE TRAIN BABY
Following a period of relative tranquility in February and much of March, a strong wave train has developed in the atmosphere, circling the globe at mid-latitudes, especially here in the Midwest. These so-called low pressure waves are well-known and a normal part of atmospheric flow. Typically, these wander around the planet, oscillating regularly between the tropical and polar regions. Once in a while, the westerlies lock into patterns where waves repeatedly form and amplify (strengthen). Active storms, slower in motion and heavier in precipitation, are the result. Occasionally, multiple storms in succession can impact specific regions such as the Midwest, enhancing the effect. From what I can see, we are on the wave train baby and another significant precipitation event is in the making. Hopefully this time, the heavier rain falls in the parts of my area that missed out recently.
Below you can see the most recent system (a three-day event), dumped much of its 2-4 inch precipitation totals west or east of my region, with the lighter amounts in eastern Iowa due to the dry slot, a spot where forcing and moisture is minimal near the track of the storm.
This beast of a storm, while having minor impacts on the dry conditions over eastern Iowa, significantly alleviated dryness over the NW half of Iowa and much of Minnesota. See the side by side graphic of this week vs. last weeks (pre-storm) drought monitor. In Minnesota alone, the amount of moderate drought went from 74 percent to 43 percent. While the majority of Iowa remains quite dry, with 71 percent in moderate to extreme drought, parts of NW Iowa are showing normal soil conditions. We need one of these major systems to cut across the SE half of Iowa to improve conditions there before we enter the growing season.
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WHAT ARE THE ODDS IT HAPPENS?
Much like last week's storm, the developing one comes out in multiple pieces, with the larger event not expected until Monday or Tuesday.
The first rain opportunity takes place Friday night and could be a sleeper for bands of heavy rain and storms. During the day, a warm front will stretch from west to east near I-80. Enough sunshine is expected to allow temperatures to surge into the low to mid 60s (perhaps upper 60s far south). The far north does not get the warm front to clear and readings will be cooler in the upper 50s.
Early Friday evening, instability and moisture are pooled along the warm front. A disturbance sweeps east along the front. At the same time the low level jet cranks up to 45 kts generating strong warm air advection. Throw in respectable shear, and the ingredients are there for rapid thunderstorm development. Additionally, freezing levels look more than adequate for hail in the stronger updrafts. The 3k NAM simulated radar shows this at 10:00pm.
Towards evening, you can see the CAPE (instability) building eastward. That's the fuel for storms when the forcing kicks in and lifts northeast.
Below you can see the helicity tracks reflective of the shear that could drive stronger hail producing storms. These cells are likely all elevated, which should keep hail as the primary severe weather threat. Some nice downpours are possible in the stronger cores.
Rainfall is going to be determined by the strength and coverage of what thunderstorms eventually develop. Some guidance is bullish, others not so much. Here's what models are suggesting for rain totals from this first event. I do think the area from about the Quad Cities east stands the best chance of seeing beneficial rains.
The 3k NAM
The HRRR
The EURO
The GFS
Once this disturbance flies by, we await another ejection of energy and warm air advection Easter. That leaves Saturday free from rain and with seasonal readings in the upper 50s to low 60s, it should be a keeper. Meantime, the front that slipped by Friday night has stalled over Missouri. Moisture begins to over-ride it Saturday night. It reaches my area Sunday in the form of clouds and some showers. Currently, the best odds of rain are showing up over the area near and south of I-80. Even here, most of the rain currently looks light, 1/4 inch or less. Some parts of the north could avoid it altogether or see little more than sprinkles. The jury is still out on how this plays out. One thing I don't like is the fact the boundary that's well south ensures east winds. If the clouds are thick and precipitation adds to evaporation cooling, Easter could be rather chilly with highs potentially holding in the 40s in spots, at best low 50s. If, the forcing is a bit further south, some filtered sunshine could allow readings in most areas to hit the 50s. We should be able to fine tune this in my next post.
THE MAIN EVENT
That leads us to the primary precipitation event, which remains very much open to interpretation. Once again, the issue at question is phasing, how much energy gets bundled into the storm from the northern and southern branches of the jet. The EURO is more split and far more progressive, sending its precipitation through the Midwest Monday. See how its 500mb structure sets up.
Compare that to the GFS below. Notice the closed 500mb low and the consolidation of energy over Iowa.
The injection of colder air creates a larger, slower moving disturbance with the potential for heavier rain and even some snow, a consistent feature with the GFS the past 3 days. Look at the mess the GFS shows Tuesday morning.
At the same time, the EURO has no rain or snow anyway close to the region as drier air has moved in.
The fact is, I've been waiting for one of these models to show a trend toward the other, and so far after 3 days it hasn't happened. Up to now, I see nothing that really tips the scales. I'm confident this is a phasing issue, and sooner or later data is going to improve to the point where one solution becomes dominate. Usually, the EURO comes out on top in situations like this, and I suspect that's where this ends up.
No matter what, Monday or Tuesday (or both) will see wet conditions. Including what precipitation falls through Sunday, here's what the models are indicating for totals Friday night onto Tuesday night.
The EURO
The GFS
Both depictions are significant, especially for spots that really need rain in eastern Iowa, Hopefully trends hold. I will say as a measure of caution, phasing really concerns me and causes models fits. If models fail to accurately predict what they are indicating now, precipitation totals could be significantly altered to lower levels. Sorry to say, my confidence level remains somewhat low after Friday night.
By the way, I doubt if we are done with snow yet this spring. Anything major is contingent on the GFS being correct, which as I mentioned seems unlikely. Here's what it shows for snow, most of it near or just north of HWY 20. I don't expect this to be the case, but I'm watching it intently.
The EURO shows some snow of its own. However, it comes from another disturbance in the NW flow Tuesday night or Wednesday morning. Maybe, maybe not.
Like it or not, we most likely have not seen the end of the white stuff just yet, especially north of I-80. April "snow" showers, yuk! I guess we'll see. That's all for now. Roll weather...TS
