How much does HPC/SPC guidance affect local NWS forecasts

[Ed Lizard]

Saw a comment, swatted down, on local office mets and whether they need to justify forecasts.

Granted, HPC guidance in particular is fairly broad brush, but there is a discussion of preferred models/model blends, and expected trends. Plus graphical surface feature and rainfall forecasts.

I understand once an official NHC forecast is out, the local forecast offices are expected to follow it fairly closely, and just add local details.

[billgwx]

We review HPC/SPC/NHC guidance and use it mostly where we see fit. The only times we're bound to follow it are of course for convective watches and tropical cyclone watches/warnings, and also when tropical cyclone forecast wind radii and/or probabilistic wind guidance suggest a potential tropical cyclone impact. Otherwise, ideally we formulate our own forecast ideas, review SPC convective outlooks, HPC QPF/Winter Weather Desk forecasts, HPC longer range guidance, etc., and along with neighboring WFO's work toward a collaborative solution.

That said, when a storm is ongoing or looming in the shorter term, I may use straight HPC long term guidance to save time and concentrate on the nearer term issues at hand--others may turn to GFS MOS or even stay with the previous forecast in a similar situation. The previous forecast is also fine with me when I think it already captures what I think will happen and I don't think I can add value to it (actually had that happen for a major winter storm a couple of years back!), or if there is enough uncertainty where I want to avoid continuity flip-flops.

[BillB]

Agreed. HPC/SPC/TPC provide good overviews of the larger scale pattern, and HPC does a good overview of which models are working well. We review these discussions routinely each shift. HPC also provides a set of gridded data that we can use (exclusively or blended) in the day 4-7 portion of the forecast.

The WFO focus is on the local level. There are times when the national view and local view do not completely agree. Case in point: there was a situation when (to our great surprise) SPC expanded a Day 1 slight risk into part of our forecast area at midday...despite our being so thoroughly capped that it would have taken all-time record heat AND increased humidity to even have a chance at breaking that cap. We locally maintained a POP of less than 15 percent. Nothing happened.

We prefer to be consistant with everyone. That is where actual collaboration between offices, when done right, can help. HPC/SPC/TPC guidence is a part of this process. But we also have to be able to justify the forecast to our customers and to ourselves.