In this manuscript, the authors present a comprehensive analysis of an extraordinary rainfall event in Portugal. In their paper they describe the synoptic situation that led to the formation of a series of cyclones across the Atlantic and the formation of an atmospheric river. They discover the sources of moisture feeding the AR and the mechanisms leading to the extreme rainfall event in Portugal. The description of the event and related processes is comprehensive and consistent.

My recommendation is to publish the article with minor corrections. The authors should revise figures 3 to 5 to improve their readability as well as revise the wording of some sentences that have been left incomplete or unclear.

Minor changes:

Line 61: Change “sup-polar”

Line 84: Change “to analyze of the spatial distribution” to “to analyze the spatial distribution”

Figure 3: Improve readability. I can not see “the thin green line inside…”. Additionally, it is not necessary to place a black contour around the highest IVT values. If retaining them, briefly explain what C1, C2 and AR0 are in the caption. What does the blue colour over the ocean mean? If possible, live open ocean white to improve figure clarity.

Lines 234-236: I assume that panels description refer to those in figures 4 and 5, please carefully state this in text if correct.

Figure 4: Please improve clarity as in figure 3. I can not see the red contour (“small inflow area”) in panel 4a, maybe masked by the IVT shaded area. Had to go big zoom to find something resembling a red contour. Can not see the inflow area (red contour) in the maximum IVT area

Lines 329-330: Change “This sequence of events can be appreciated in our results where we observe that the C1 system, to which the AR was associated following the C0 system’s path.” I can not understand the meaning of this sentence.

Lines 338-339: Rewrite, the sentence does not seem complete

General Comments

In general, this paper is a good contribution assessing the synoptic conditions of a really impactful precipitation event affecting the Iberian peninsula. Overall I think this paper is a good contribution but could be made stronger and clearer. I hope that my suggestions are helpful.  I think it is important for this paper to provide some additional context and motivation for the case study in addition to noting the impactful nature of the event. I also suggest that the sections could be reframed a bit for clarity – e.g. sections 3-5 could be combined as delving into the case study overall with subsections for the pre-conditions, AR evolution, and resulting precipitation (in that order).

Specific Comments

Section 1 – could you mention how many ARs affect Portugal each year? (line 64) and how much precipitation input they provide? Can you also define a little better what you mean by “inter the evolution of rainfall” – like trends throughout the period? Maybe mention this? Also, why give the top three rainfall events from 1863 to 2013 instead of in the whole record? What are the incidents reported by the Portuguese Civil Authority – are they rescues, or damage reports, or both? I think maybe some more clarity and context in this part of the Introduction would benefit the study. I suggest also when you define the goals for the paper in the last paragraph of the section, you could remind the reader again why this case study is important to understand – because it was so impactful, of course – but could it be better predicted if it was better understood? Are we expecting more events of this nature more frequently? Any other relevant literature on frequency of extremes or information on the predictability that could be included in Section 1?

Line 111-  I don’t think this sentence is necessary – I’m not sure how you would justify not including the whole column or where you would stop computation.

Section 2.1 – I am not clear on why not use the entire IDL daily precipitation record.

Section 2.2 – Is there any additional justification on the detection methodology other than its wide use? What about it makes it so widely applicable and in particular applicable here?

Section 3 – what is the main takeaway from the spatial distribution? Is it the widespread nature of the event? There is also some discussion here about the record-breaking nature and percentile value associated with the given events. Consider keeping the discussion narrower and focused on the spatial distribution and what we are meant to take away from it, or perhaps provide all the same information (in a table?) for different stations that have had their records broken and include the percentile information, so we are getting the spatial sense of that as well.

Line 170 – add reason behind starting to evaluate the synoptic conditions on 5 December and SSTs on 1 December.

Figure 2- add units for a/b color bars.

Section 4 – are the noted patterns e.g. in Figure 2 usual patterns for extreme events? Or what other meaning should the reader take from the evaluation?

Line 212- what is considered relatively intense IVT? Over 500 units? Consider adding this information. Also – should “suggest” be “hypothesize”? (similar note to “probably” in Line 218). What would be needed to test these hypotheses (even if not in scope of this paper)?

Line 242 – should “intensifying” instead be something like “supplying”?

Line 250 – should this be that the IVT as a whole is still high in terms of both moisture and winds?

Line 258 – I think the first e.g. should be something like “than in locations like California with significant orography at the coast”. The authors are stating that dynamic forcing is more important right? Perhaps state that explicitly and then note that you will evaluate that forcing.

Figure 6 – could the same map view be used for both columns?

Line 289-290 – I think this is an incomplete sentence.

Section 6 – similar comments to the introduction – I think placing these results in the context of what we can do with them is important. The conclusion here is more of a summary, which is fine, but I think it would benefit from more discussion on implications and utility of the results. Some of this is in the last paragraph but I think it should be highlighted throughout the paper, along with information about what is needed to test the hypotheses mentioned in the paper.

Line 338-340 – This sounds like an incomplete sentence, please rephrase.

Line 344 – rephrase “but affecting central Portugal in an extremer way” to something like “with the most extreme impacts/rainfall in central Portugal”

Technical Corrections

A general quick read for grammar/word choice (clarity)/readability is warranted although generally the paper is in good shape. A few suggested changes are below (non-exhaustive).

Line 15 – remove “an” before “above normal”

Line 45 – I think you mean to remove “may lead to EPEs” given the rest of the sentence

Line 56 – should be “resulting in” instead of “resulting on”

Line 62 – I think you can remove “multiple” as it is redundant

Line 74 – introduce what is “IPMA” here instead of later in the paper

Line 75 – I think you could put this footnote instead on Line 66 when noting the daily data record.

Line 132 – consider changing “nowadays” to in recent years

Lines 225/229 – “lead” should be “led”

Line 354 – should be “inevitably”