Matt Makens

For much of the region June's weather pattern will remain for July, however, a
wetter change is possible for the Northern High Plains.

Let's start with June's data.

Much of the Central United States has been warmer than average this month, as
well as drier than average. This is to be expected, in previous posts we've
discussed at length the overall ocean-atmosphere teleconnections that spell this
out for us. 

Below we see temperature anomalies over the last 28 days, showing temperatures
well above average for our entire region, with some cooler than average
temperatures across the Pacific Northwest and Southeast United States. Denver,
through the first 28 days of the month, is averaging 3.8°F above average for
June, and thus will end the month well above average. The same story is true for
most of the Central and High Plains: 

For precipitation, plenty of dry to be found for us as well. Some spotty
wetter-than- average areas across Colorado, with pockets of more pronounced
wetness across Kansas, but overall the drier than average pattern we have been
concerned about has certainly been in place: 

The drought also grew notably during the month of June. There was a period of
record heat and strong wind that created a flash drought situation across the
plains as discussed in Colorado's Assistant State Climatologist Becky
Bolinger's
post
[https://www.weather5280.com/blog/2020/06/24/have-you-heard-the-term-flash-drought-what-exactly-does-that-mean/]
. 

Between May 26th and June 23rd we saw the area designated by the U.S. Drought
Monitor has experiencing no abnormally dry or drought conditions drop for 60%
across the High Plains to 37%, with a large swath of Wyoming and the Dakotas
being added to the D1 Moderate Drought class: 

For the most part we haven't discussed the Madden-Julian Oscillation (MJO) cycle
and it's connection to our local pattern in June. Let me bring that forecast in
now.

The MJO as modeled for early July hasn't changed from what it was largely doing
in June and that is to hang out in between Phases 1, 2, and 3.

What that means for precipitation is shown below. (These composites are centered
on July, but are a close match to those of June as well).

The phases 1 to 3 will largely favor the Northern U.S. and, at times, the
Central Plains with moisture. Those same phases didn't quite bring that result
to June as shown above with the June precipitation map. If we can get an
amplified phase 2 for any amount of time in July, perhaps that would help bring
more moisture to those areas that need it most. 

Perhaps one, of many, factors that decreased the MJO effectiveness in June was a
positive phase of the East Pacific Oscillation (EPO). 

A positive EPO translates to warmer than average conditions across the entire
Lower 48. A negative EPO translates into warmer than average conditions shifting
west allowing cooler air to impact northern parts of the Lower 48.

Looking at the EPO forecast for early July shows a negative trend, something we
haven't seen in awhile and could mean, in combination with the MJO phase, a
wetter and potentially cooler northern US while conditions last.

So, with that said let's look at July-modeled data. First the European ECMWF,
then the American CFS. In each case we will look at comparing the precipitation
guidance of both and then comparing the temperature guidance.

Overall, a similar 'vibe' with the highest precipitation expected across the
Deep South and for some of the Northern Plains. However, CFS is more generous
with rainfall for Western U.S. and Colorado, and happens to look much more like
a Phase 2 MJO vs the Euro which looks closer to a Phase 1 and 3 MJO. It'll be
interesting to see if the CFS is onto something or not. 

For temperatures, below, there is general agreement to cooler than average
conditions for the Northern High Plains and mostly heat elsewhere.

Note that the temperature comparison is that of °F on the Euro to °K on the CFS. 

Comparing those model outputs to that of analogous years (most similar
ocean-atmosphere connections to the present), the precipitation years do not
show a favored wetness to the north. So much of July precipitation falls as
thunderstorm complexes, it makes sense, in my opinion, to see it so
inconclusive.

Temperature anomalies agree to the heat, but not so much on cooler air.

However, let's look at those analogous years that also had a significantly
negative EPO index value as we are expecting for this July. What you'll see is
much more inline with the Euro forecast.


Bottom Line
The teleconnections indicate a cooler than average July for much of the northern
U.S. as is further verified by analogous years and forecast models. The
remainder of the country, namely southern plains and western US will likely be
warmer to much warmer than average.

Per precipitation, there's really no reason to go against what has happened in
June. One of the few factors that has changed between June and July is the EPO
and that's more of a temperature impact, not precipitation. Outside of
significant thunderstorm complexes, there is no precipitation pattern shift for
the month that we see at this time. Drought areas are not likely to receive
enough water to slow or stall the drought. 

If the MJO could swing into Phase 2 and we see the northern Pacific trend
negative, we would have a reason to look forward to more water. Right now the
MJO and EPO appear to show some signs of wanting to better cooperate in July and
June, but the signal for wet just isn't there as of now. We will continue to
watch.

July outlook: temperature pattern to shift but precipitation likely to remain much the same

Denver, CO